automatically building BUILD from code

[johnynek]

seems pants has a plugin for this:

https://github.com/foursquare/fsqio/tree/master/src/python/fsqio/pants/buildgen

we could possibly pilfer this.

[johnynek]

This could be useful:

https://github.com/lightbend/scala-sculpt/blob/master/src/main/scala/com/lightbend/tools/sculpt/plugin/ExtractDependencies.scala

the above code can print all the dependencies of some scala. So, we could use this to emit meta-data for each target on how you could split a target (which files/jar deps need to go into each cluster).

This could be done with a goal of keeping some target compilation time (so don't make tiny targets if a slightly larger one wouldn't measurably increase compilation time).

[johnynek]

lightbend-labs/scala-sculpt#61

Related PR to build a tool to split big targets automatically.