Check import selectors at Typer

[odersky]

Import selectors were checked at PostTyper before, but that can lead
to confusion when a type error results from a bad import selector.

Fixes #15468

[som-snytt]

The tests show this is a good change. There was a pre-pandemic ticket on Scala 2 to refcheck imports: the use case was an unused import where the import was deprecated, but the deprecation is not reported. I tried it out here and noticed that importing from a deprecated thing is not reported in Scala 3. I'll create a ticket.

[som-snytt]

It's not obvious to me that one of these is more wrong.

  import X.{x => a, x => b}
  import X.{x => a}, X.{x => b}

If anything, the first form of aliasing is less wrong because the duplication is obvious. In the second form, or if the second import is much later in the source, the duplication is probably an accident; an exact duplicate would be benign shadow.