Fix recursive globs through symlinks

filepath.Walk does not walk symlinks to directories, which leads to
inconsitent behavior with following symlinks.  Replace the use of
filepath.Walk in ListDirsRecursive with a helper function that lists
each directory and walks anything for which IsDir reports true, which
includes following symlinks, and then reconstructs the path through
the symlink.

Test: fs_test.go
Test: glob_test.go
Change-Id: Ie4dd0820e9c7c0a124caa65210ce20439a44da16