Rewrite pathtools.Glob to track partially-matched directories

The directory structure:
  a/
    a
  b/
    b
With the glob pattern */a would previously return []string{"a"} for
dirs, but it needs to return []string{"a", "b"} in order to re-run
the generator if a file called "a" is created inside b/.

Rewrite Glob to manually recurse through path elements, only calling
filepath.Glob for a pattern with wilds in the last element of the
path, and add the globbed directory to the dirs list each time.

Also add tests and test data for pathtools.Glob.

Change-Id: Ibbdb2f99809ea0826d4fa82066cf84103005ef57