[FR]: Support proto_library in other packages for ts_proto_library

### What is the current behavior?

Right now `ts_proto_library` only supports using depending on `proto_library` in the same package as it requires `proto_srcs` to be set, which takes file names as opposed to target names. This makes it impossible to include dependencies. See https://github.com/aspect-build/rules_ts/discussions/627 for an example.

### Describe the feature

`ts_proto_library` should be able to depend on protobuf messages and `proto_library` rules defined in other packages, and `copy_files` should copy them to the right place.

Start by tracing how ts_proto_library consumes proto_srcs and how copy_files handles generated dependencies. Reproduce the cross-package case from discussion 627, then compare same-package and other-package proto_library targets. Done means protobuf messages and proto_library rules from other packages work and copy_files places them correctly.

Written by the indexing model from the issue text.