scalapb_proto_library doesn't support custom TypeMappers anymore?

Hi!

After switching on aspect-based protobuf compilation (#700) it seems like it's no longer possible to use custom `TypeMapper` from ScalaPB. At the first glance the reason is that bazel tries to generate scala code from `proto_library` (`%name-fast` rule) but there is no way to specify scala libraries for `proto_library` (at least I haven't found).

It feels like it used to work before because previously bazel was using `scalapb_proto_library` with its `deps` to generate final scala code, where you can specify additional custom classes.

I've also prepared [small example which reproduces this issue](https://github.com/sholokhov/bazel-scalapb-example).

Bazel version - `0.25.2`
rules_scala - `b2e12827240fc488bbe5ebbbbcbc7a4f12ca02f2`

Start with the linked bazel-scalapb-example and compare the aspect-based compilation introduced in #700 with the scalapb_proto_library path. Inspect how %name-fast is generated from proto_library and how custom TypeMapper classes are supplied through deps. Done means the example can use a custom ScalaPB TypeMapper with aspect-based compilation.

Written by the indexing model from the issue text.