Stop assuming deps of go_proto_library has importmap attribute (#4189)

**What type of PR is this?**

Bug fix

**What does this PR do? Why is it needed?**
`_go_proto_aspect` applies to all `deps` of `go_proto_library`
transitively, following the `deps` and `embed` attributes. Meanwhile,
the only requirement for `deps` of `go_proto_library` is to provide
`GoInfo`. They don't have to be `go_proto_library` or `go_library`. We
should not assume they also have `importmap`, `importpath_aliases` or
`embed` attributes. They are likely to have `importpath`, otherwise
Gazelle won't be able to resolve them. So we can keep the assumption of
`importpath` for now until breakages are discovered.


cc @dzbarsky