scalapb_proto_library additional scala sources

Now it is impossible to add some custom scala sources into scalapb library. So i could not do something like this:

trait TagRecordCompanionTrait {
    implicit val ordering: Ordering[TagRecord] = (x: TagRecord, y: TagRecord) => {
     if (x.count < y.count) 1
     else if (x.count > y.count) -1
     else x.name compareTo y.name
   }
}

message TagRecord {
    option (scalapb.message).companion_extends = "TagRecordCompanionTrait";

    required string name = 1;
    required int64 count = 2;
}

The only way to do this is to use preamble in proto file

Start at the scalapb_proto_library rule and compare its current source inputs with the preamble-based workaround described in the issue. Trace how custom Scala sources are passed into the generated library, then verify that a companion extension such as TagRecordCompanionTrait can be supplied without using a proto preamble.