Handle .proto files in different package for filegroups

Followup to aosp/2693190, this CL adds the support to filegroups.
<mod>__bp2build_converted is now an alias to a proto_library
target <mod>_proto. This proto_library will be created in a different
package if the .proto file exists in a different package.

Test: bp2build unit tests
Test: TH
Bug: 292583584
Change-Id: I8ca452aacf1a86dfc9e218464e38aab89afa5a29