Add all-named-(dirs|files)-under and related

To consolidate the number of places that we're using 'find' in the tree,
add some more helpers:

  all-named-dirs-under
  all-subdir-named-dirs
  all-named-files-under
  all-subdir-named-files

This change also makes many of the current helpers use these helpers
instead of using their own implementation.

The 'dirs' helpers are using '-type d' so that they only output
directories. It's probably safe to use '-type f' for the files helpers,
but that increased the kati load time by ~20%.

Bug: 24204119
Change-Id: I3312e2fe8c146f10955e1d986ad15d9c8be494e1