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
Add support for specifying symlinks in mock filesystems to prepare
for glob symlink tests.
This patch leaves incorrect behavior by not walking symlinks in
mockFs.ListDirsRecursive, but it matches what osFs does.
Test: fs_test.go
Change-Id: If87a83c00f21e14696faf890b7b09e88b18e95b9
