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

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.

src/compiler/scala/tools/nsc/ast/parser/Parsers.scala

@@ -2700,7 +2700,16 @@ self =>
      *  }}}
      */
     def importSelectors(): List[ImportSelector] = {
-      val selectors = inBracesOrNil(commaSeparated(importSelector()))
+      val selectors0 = inBracesOrNil(commaSeparated(importSelector()))
+
+      // Treat an import of `*, given` or `given, *` as if it was an import of `*`
+      // since the former in Scala 3 has the same semantics as the latter in Scala 2.
+      val selectors =
+        if (currentRun.isScala3 && selectors0.exists(_.isWildcard))
+          selectors0.filterNot(sel => sel.name == nme.`given` && sel.rename == sel.name)
+        else
+          selectors0
+
       for (t <- selectors.init if t.isWildcard) syntaxError(t.namePos, "Wildcard import must be in last position")
       selectors
     }

test/files/pos/import-future.scala

@@ -26,7 +26,7 @@ class C {
 
 object starring {
 
-  import scala.concurrent.*, duration.{Duration as D, *}, ExecutionContext.Implicits.*
+  import scala.concurrent.{*, given}, duration.{given, Duration as D, *}, ExecutionContext.Implicits.*
 
   val f = Future(42)
   val r = Await.result(f, D.Inf)