ScalaPB additional args

[DanielBlanco]

@lefou can you please review, and let me know if I'm missing something?

The plan with this change is to allow additional args for scalaPBC, this means I can just do this in my code for zio-grpc to work:

object example extends ScalaPBModule {
  def scalaVersion = "2.12.6"
  def scalaPBVersion = "0.7.4"

   override def scalaPBAdditionalArgs = Seq(
        s"--zio_out=${T.dest.toIO.getCanonicalPath}"
   )
}

[lefou]

Your pull request looks good to me. Personally, I'd name the new target "additionalArgs" instead of "customArgs" as that better transports the fact that other args logic will still be preserved and it also matches what we have in other plugins, but that's only a suggestion and up to you.

But I'd like to have some more ScalaDoc, at least on the newly added and changed targets. These will help any user of the plugin who edits their buildfile in an IDE.

[lefou]

Thank you, @DanielBlanco !

[chikei]

Actually this PR might break ScalaPB module as ScalaPB module append generatedSources with compileScalaPB which was the scalapbc output path, but here compilationArgsScalaPB incorrectly write output path using T.dest from compilationArgsScalaPB. @DanielBlanco

You can test these by uncomment test calledDirectly.

[DanielBlanco]

OK, I'll try the test and come up with a solution if I can. Thx @chikei.