SemanticDB phases

Stripe's Scala team has an intern, and this quarter we're planning work that requires SemanticDB outputs!

I believe SemanticDB support can be easily encapsulated in a new optional phase that:
1. adds the scalac plugin
2. declares the new SemanticDB output files

This pattern was already prototyped in higherkindness/rules_scala, so I'm confident that it will work here too.

We'll circle back soon with an update once we can commit to this work.

Review the repository's existing phase implementations and compare them with the higherkindness/rules_scala prototype referenced in the issue. Done means an optional phase adds the scalac SemanticDB plugin and declares the resulting SemanticDB output files.

Written by the indexing model from the issue text.