Configure Idea project generator per module

[lefou]

This PR is a first sketch how to skip some modules in Idea project generator.

The motivation is simple:

• Idea cannot handle multiple projects with the same base directory
• When working with Cross to enable cross compile projects of any kind, Idea selects one of the cross versions (almost) randomly per directory, which is undesired

This is a quickly hacked PR to sketch one possible solution to tackle the need to customize project generation. I'm open for input / change requests. If we can reach a consensus about the general direction, we can beautify, e.g. move the IdeaConfigModule into it's own file. Also tests would be nice, but I'm not familiar with the integration test setup of mill yet.

[lihaoyi]

I'd be happy just hard-coding the intellij project generation flag inside JavaModule, since the intellij project generation is already hard-coded to work with JavaModule targets anyway

[lihaoyi]

If some day we generalize IntelliJ project generation to work for other arbitrary modules (python, js, ...) we can move the flag then

[lefou]

What about "grouping" idea related def's by trait JavaModule extends IdeaConfigModule?

[lihaoyi]

Currently idea-related defs aren't standalone; they live mixed in with the JavaModule defs, as bits and pieces refactored out to be conveniently consumed by intellij. Maybe in future when we have more idea-related stuff we can extract it out into it's own trait

[megri]

This could use a collect instead to avoid iterating over the modules twice

.collect {
  case (path, mod) if !mod.skipIdea => moduleName(path)
}