Support scala file parsing prior to gazelle runtime

[pcj]

This PR adds a new rule scala_files and related scala_fileset rules:

• a scala_files rule aggregates the files named by existing scala rule srcs globs. There is one scala_files rule per scala package.
• the scala_files output is a file.Rule
• scala_fileset aggregates / merges multiple scala_files into a file.RuleSet
• runs as a persistent worker

The file.RuleSet can be fed into the source provider as a gazelle argument that will obviate the need to parse the file at runtime.