Fix the issue that changing merge_annotations_dir doesn't trigger build

We don't have API exported to glob any dir outside of current module.
Any files deps outside of current module should be either a
filegroup/filegroup or a customized module.

We already have similar customized module to track droiddoc-template, so
rename it to be more generic so that it can be used by
merge_annotations_dir also.

Bug: b/111916275, b/70351683
Test: touch manual/android/support/design/widget/annotations.xml, and m
-j metalava-api-stubs-docs, and check the built output srcjar.

Change-Id: I75420ddba69785e46bea75b6dd3f189be7cfe5ad