I keep meaning to take the time and implement this. +1.

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/a10ef48a41fb59facfdad9d3874ff4418b98a743

commit a10ef48a41fb59facfdad9d3874ff4418b98a743
Author: Robert Iannucci <iannucci@google.com>
Date: Thu Aug 17 16:45:47 2017

[cipd] Move template expansion code to common.

This will enable us to expand packages in other contexts (such as
command-line parameters).

This also fixes subdir expansion to behave more like package name
expansion; it uses the expansion paramaters passed to ResolveWith,
and if a subdirectory uses the ${param=value} syntax and doesn't
match the host, the entire subdirectory is skipped.

R=vadimsh@chromium.org

Bug:  756288 
Change-Id: Ic9f9bf9fb0f8e64a45bc99c30d3a78f6c8acbb49
Reviewed-on: https://chromium-review.googlesource.com/618302
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>

[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/common/common.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/common/common_test.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/ensure/bad_test.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/ensure/doc.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/ensure/file.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/ensure/good_test.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/ensure/item_parsers.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/cipd/client/cipd/ensure/package_def.go
[delete] https://crrev.com/2385d6a3a01def02d56d98ce3afbad2e77ab61f5/cipd/client/cipd/ensure/template.go
[modify] https://crrev.com/a10ef48a41fb59facfdad9d3874ff4418b98a743/vpython/cipd/cipd.go

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/91b1bda5d4f6bd552591715cad9e2d0eb887be4e

commit 91b1bda5d4f6bd552591715cad9e2d0eb887be4e
Author: Robert Iannucci <iannucci@google.com>
Date: Thu Aug 17 18:05:38 2017

[cipd] Enable package name templates for all CLI functions

Also fix up the ensure file format documentation to look nicer in godoc.

R=vadimsh@chromium.org

Bug:  756288 
Change-Id: I910906efe25c07f57ffd2e3f7226552b0653ba1c
Reviewed-on: https://chromium-review.googlesource.com/619274
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/91b1bda5d4f6bd552591715cad9e2d0eb887be4e/cipd/client/cipd/ensure/doc.go
[modify] https://crrev.com/91b1bda5d4f6bd552591715cad9e2d0eb887be4e/cipd/client/cli/friendly.go
[modify] https://crrev.com/91b1bda5d4f6bd552591715cad9e2d0eb887be4e/cipd/client/cli/main.go

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/cf5b0de14e94b06274d15e84b19449f580576e80

commit cf5b0de14e94b06274d15e84b19449f580576e80
Author: Robert Iannucci <iannucci@chromium.org>
Date: Thu Aug 17 22:31:28 2017

Roll go.chromium.org/luci

infra/go/src/go.chromium.org/luci:
e130567e [cipd] Add spacers to help text to make it more readable.
91b1bda5 [cipd] Enable package name templates for all CLI functions
050e243c [gaeauth/server] Split off OAuth2 method.
a10ef48a [cipd] Move template expansion code to common.
2385d6a3 [gaeauth] Use server package request cache.
12430035 [server] Add request and global caches.
0777502c [logdog] Update Storage caching mechanism.
9eb5e16a [logdog] Update Storage to take Context.
63d021f6 [luci_config/appengine] Add flex config.
846eeb92 tokenserver: Support certificates with SAN field.
7902d31a Regenerate Swarming API endpoints wrapper.
8438d5e5 tokenserver: Allow callers to supply more information for the BigQuery log.
297f4da2 Milo: Fix passing in server to swarming build/log pages
cedf0b3a scheduler: Extract some chunks of engine.go into separate files.
cdc32b71 Milo: Copy source manifest proto over
5f32920e lucictx: Add 'SwitchLocalAccount' function to switch default account.
11a2b13c tokenserver: Log generated OAuth tokens to BigQuery.
2c80f717 Add ViewURL to config metas.

TBR=dnj@chromium.org, vadimsh@chromium.org

Bug:  756288 
Change-Id: Iaa5ff48785a0fc190abc384f99dfebbdeedfa6ab
Reviewed-on: https://chromium-review.googlesource.com/619601
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/cf5b0de14e94b06274d15e84b19449f580576e80/DEPS

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/depot_tools/+/6f1a51452aaf210f00b6bb765bd9eb6b28e8efb3

commit 6f1a51452aaf210f00b6bb765bd9eb6b28e8efb3
Author: Robert Iannucci <iannucci@chromium.org>
Date: Thu Aug 17 23:30:59 2017

Bump cipd client to 1.7.1

e130567e [cipd] Add spacers to help text to make it more readable.
91b1bda5 [cipd] Enable package name templates for all CLI functions
a10ef48a [cipd] Move template expansion code to common.
4c613c75 cipd: Support template expansion in @subdir
3847616b Rename github.com/luci/{luci-go,gae} to go.chromium.org/{luci,gae}.

R=vadimsh@chromium.org

Bug:  756288 
Change-Id: Ie40daf2d80ea715fb303eb199b6c5abf8b4870ef
Reviewed-on: https://chromium-review.googlesource.com/619966
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/6f1a51452aaf210f00b6bb765bd9eb6b28e8efb3/cipd_client_version

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/34a2baa5c096819ad083e6decd5ba51fdb3d9ade

commit 34a2baa5c096819ad083e6decd5ba51fdb3d9ade
Author: Robert Iannucci <iannucci@chromium.org>
Date: Thu Aug 17 23:43:21 2017

Bump cipd staging version

e130567e [cipd] Add spacers to help text to make it more readable.
91b1bda5 [cipd] Enable package name templates for all CLI functions
a10ef48a [cipd] Move template expansion code to common.
4c613c75 cipd: Support template expansion in @subdir
3847616b Rename github.com/luci/{luci-go,gae} to go.chromium.org/{luci,gae}.

R=dnj@chromium.org, vadimsh@chromium.org

Bug:  756288 
Change-Id: I71f9ef9e4be6ffa30baff957336e9ac98d8b8d6b
Reviewed-on: https://chromium-review.googlesource.com/619986
Reviewed-by: Vadim Shtayura <vadimsh@chromium.org>
Commit-Queue: Robbie Iannucci <iannucci@chromium.org>

[modify] https://crrev.com/34a2baa5c096819ad083e6decd5ba51fdb3d9ade/scripts/slave/cipd_bootstrap_v2.py