Replace import $file syntax with flat package namespacing

[lihaoyi]

Fixes #4459
Fixes #3894

With this PR, all .mill files in a folder are in the same flat package namespace, similar to how normal Scala files work. The previous import $file syntax was inherited from Ammonite, and unfortunately never took off across the Scala community. In effect this makes .mill files behave almost exactly like .scala files, which is part of the longer term goal of removing special handling for .mill files.

The code generation for the helper files is not changed at all in this PR, rather we just make use of Scala 3's export clauses to make them available under the main build.mill/package.mill file as well

There is the caveat that the package objects in .mill files can be referenced via build.foo.bar without the trailing .package. Implementing this caveat is the cause of a lot of the hackiness in the codegen, which will hopefully go away once the upstream language feature lands scala/improvement-proposals#100

We leave the codegenned import _root_.{build_ => $file} in place so import $file will continue to work to ease in the migration, but are no longer the recommended way of referencing helper files