java_sdk_library: Access outputs using tags

Previously, in order to access say the public stubs source jar it was
necessary to directly reference the module by name. This change adds
support for accessing them indirectly via tags.

Test: nothing
Bug: 155164730
Merged-In: Id6d76e56c7b46944b2d2a44a2163fb05a5b03de9
Change-Id: Id6d76e56c7b46944b2d2a44a2163fb05a5b03de9
(cherry picked from commit 46dc45ab)