Ensure Mill names are only exported from one top-level package

[lihaoyi]

Right now, importing both javalib._ and scalalib._ results in an error due to ambiguity when the same classes are exported from both:

package build
import mill._, javalib._, scalalib._

val x = Assembly

#06 [info] compiling 1 Scala source to /Users/lihaoyi/Github/mill/example/scalalib/basic/1-simple/out/mill-build/compile.dest/classes ...
#06 [error] /Users/lihaoyi/Github/mill/example/scalalib/basic/1-simple/build.mill:4:9: reference to Assembly is ambiguous;
#06 [error] it is imported twice in the same scope by
#06 [error] import scalalib._
#06 [error] and import javalib._
#06 [error] val x = Assembly
#06 [error]         ^
#06 [error] one error found

This isn't much of a problem now, as scalalib is a strict superset of javalib, so you can just use that all the time. But it may become a problem when #3567 breaks out javalib from scalalib so that they may each have their own unique features, and #3451 introduces kotlinlib._. When a user tries to have a multi-language codebase like import javalib._, scalalib._, kotlinlib._, they will inevitably hit the errors above

A solution to this would be to ensure that the different *lib packages do not export things that their upstream dependencies already export. So e.g.

• If scalalib depends on javalib, you would need import javalib._, scalalib._ to work.
• And similarly if kotlinlib depends on javalib, import javalib._, kotlinlib._.
• And for a project with both java/scala/kotlin, import javalib._, kotlinlib._, scalalib._ would work without ambiguity

This will involve moving/removing a bunch of forwarders, and could happen together with #3567 in 0.13.0