go_proto_library: report errors for unknown imports

[jayconrod]

go_proto_library may depend on go_proto_library and go_library targets (anything that provides the GoLibrary provider). If a go_proto_library depends on a go_library built from .pb.go files that were pre-generated from some .proto file, it will not know how to map proto imports to the go_library. That information is communicated through the GoProtoImports provider, generated by the go_proto_aspect.

We should report an error (or at least a warning) whenever we're building a proto library and we don't know how to map an imported .proto to a Go import path.