Integrate referenceable package objects in Mill's codegen

[lihaoyi]

• SIP-68: Reference-able Package Objects scala/improvement-proposals#100

It has landed in Scala 3 upstream. It should in theory simplify our codegen significantly, allowing us to do away with the hacky "fake packages and import/export forwarders" we're generating to try and fake the feature via codegen.

[lihaoyi]

Looks like this won't be non-experimental until Scala 3.8.x earliest, so we won't be able to make use of them in Mill 0.13.x