Depend on a module instead of real dir for metalava_merge_annotations_dirs

Bug: b/111916275
Test: touch manual/android/support/design/widget/annotations.xml, and m
-j metalava-api-stubs-docs, and check the built output srcjar.

Change-Id: I82e8b51d66f97ebc3f9f4f5aa75ce33066c95089