Within a BUILD.gn file, I can't use `gf` in Vim to move to a dependency given by an absolute path (relative to source root of course).

From chrome/browser/BUILD.gn:

1. Move cursor to "//mash/common"
2. `gf`

Expected: opens //mash/common/BUILD.gn
Actual: opens //mash/common (the directory)

Reduced case:

 :echo gn#TranslateToBuildFile("foo/bar")

Expected: "foo/bar/BUILD.gn"
Actual: "foo/bar"

Workaround: Install the google-vim/maktaba plugin, AND call `:runtime autoload/maktaba.vim`.

I think there are 2 bugs:

1. The fallback (without maktaba) uses `isdirectory()`, which only works for a relative directory if the directory exists relative to the current directory.

2. autoload/gn.vim checks for maktaba, but merely checking for a function's existence does not cause the vim file defining the function to be autoloaded. Thus the call to `:runtime` is necessary to load maktaba before calling gn#TranslateToBuildFile.



note: gn/misc/vim/ftplugin/gn.vim sets `includeexpr` to gn#TranslateToBuildFile, which causes it to be called when looking for a file when the buffer is a .gn file