Exclude synthetic this.m, Any.m from import lookup

[som-snytt]

Follow-up to #22633 which restricts import prefixes to stable prefixes.

This commit further excludes this and super; these are arbitrary synthetic expressions introduced by macros.

Noticed at #22690 (comment) which now warns correctly.

Requires a unit test.

Fixes #22690

[som-snytt]

The "fix" has an interaction with variable assignment. By chance, this.x = 42 would not record a reference.

How did that ever work? It gets an attachment on the way in. (So it wasn't by chance after all.)

  override def prepareForAssign(tree: Assign)(using Context): Context =
    tree.lhs.putAttachment(Ignore, ()) // don't take LHS reference as a read

Check for Ignore before refUsage.

[som-snytt]

A simple macro doesn't emit this.eq with a synthetic position, so a unit test requires further "research".

It would be helpful to have "integration" tests that allowed sbt builds or scala-cli builds. That would simplify use of "tests with dependencies". In real terms, the CB tests already enforce that circe "just works". Maybe sbt build tests would be restricted to dependencies from the CBs.

[som-snytt]

I'll follow up with a unit test after reducing the circe example.