dirmods: Update to take absolute paths

Dirmods currently expects a path relative to the top of the build. If
an abs path is passed, it will never match anything.

Modify so that if path is absolute, we fix-up the path relative to
ANDROID_BUILD_TOP.

Test: manual
Bug: trivial
Change-Id: Iad99228078dc693ab5f7957e007fe1bd5cea07d2