Allow import x.{*, given} under -Xsource:3

[smarter]

Note: This PR sits on top of #9722 but could be made independent if needed.

---

Imagine a Scala 3 library containing:

object A {
  val a: Int = 1
  given b: Int = 2
}

To import all members of A from Scala 2, we write import A.*,
but to do the same from Scala 3, we need to write import A.{*, given}
instead. This complicates cross-compilation for projects which depend on
Scala 3 libraries (unless these libraries exclusively use implicit
which is not something we want to encourage).

This commit remedies this by allowing import x.{*, given} (and import x.{given, *}), this is easy to do since we can just pretend the user
wrote import x.* which will give us both regular and given members in
Scala 2 code and therefore match the semantics of Scala 3.

[dwijnand]

What about import x.{foo, *, given}, when you need to name foo to give it higher precedence in name shadowing? You can split into two imports, but perhaps it would be better to avoid the problem?

Other than that LGTM.

[smarter]

That should work too

[dwijnand]

Oh, I guess that already works ("duration.{Duration as D, given, *}"). But are those two meant to always be next to each other? What if I like alphabetic ordering and import x.{given, someNameAfterG, *}?

[dwijnand]

Still curious, but less concerned.

[smarter]

But are those two meant to always be next to each other?

That's required by this implementation yes, doing something more general is a bit tricky because we need to avoid triggering the deprecation warning for given as an identifier at the same time

[smarter]

doing something more general is a bit tricky because we need to avoid triggering the deprecation warning for given as an identifier at the same time

#9722 was changed to not warn when referring to an identifier called given, so I've been able to simplify this PR and make it work with import x.{given, someNameAfterG, *} too.