Especially update.py has become pretty complicated and is due for a clean-up.

My main idea is to refactor update and package.py into three scripts:

1. update.py
   Responsible for downloading/updating only, suitable for running as a hook.
   It should also be able to take a parameter to update the alternative packages
   that currently have separate download scripts in tools/clang/scripts/
   Should also keep in mind that some non-Chromium users rely on this script
   for getting a fresh Clang. Keep supporting that use-case and see if we can
   make it simpler.

2. build.py
   Break out the "build clang" part from update.py into this script.
   The current build is complicated, see if we can make it simpler.
   See if we can make it faster e.g. by using goma for the stage-1 build.

3. package.py
   Takes the output from build.py and creates the tarball, similar to
   today's package.py.