Ignore empty module.sc files when generating code

[lihaoyi]

We want to keep them as markers to limit how deeply in the folder hierarchy we discover things, so we don't need to traverse the project arbitrarily deeply every time. But we want to allow people to migrate individual sub-sub-folders to module.sc without causing naming conflict with modules defined in the project root.

This allows us to use empty module.sc files as markers for discovery but avoid generating code for them, avoiding the conflicts

[lefou]

What about spaces and comments?

I think it's reasonable to allow build maintainers to document the purpose of a file:

// file: module.sc
// marker for mill to search for sub-modules in this directory

or even leave some commented out stuff in it

// todo: finishe migration
// import mill._, scalalib._
// object foo extends ScalaModule 

[lihaoyi]

good point. For that to work robustly, we would likely need to filter it out after the parser. We could probably make our existing compiler plugins perform that filtering

[lefou]

Sometimes perfect is the enemy of a working solution. Let's ignore comments on a best effort basis and improve later.

Suggested change

	.filter(p => p.last == "module.sc" && os.read(p).nonEmpty)
	.filter(p => p.last == "module.sc" &&
	!os.read.lines(p).map(_.trim())
	.forall(l => l.isEmpty || l.startsWith("//"))

[lefou]

@lihaoyi WDYT?

[lihaoyi]

@lefou let's hold off on this a bit, I suspect #3426 will change how this needs to be done

[lihaoyi]

Superseded by #3426