Restore resolving prefixes of implicit Ident

[som-snytt]

Fixes #22744

This code was originally for derived, but is also needed for implicits (where private objects are in the path).

#17095

[som-snytt]

During the "refactor" (there was refactoring before it was squashed into a rewrite, aka a refactor with behavior changes), I tried to preserve institutional knowledge. I still refer to history in considering new tickets. I miss having history of my own, although I've preserved the unsquashed branch somewhere. (I used to refer to "the August code", "the October code", "the November code".)

I had either missed or forgotten the classic: Refactor unused imports to try and make sense of it.

[som-snytt]

It induces spurious warnings by virtue of aliasing at different position?

-- [E198] Unused Symbol Warning: tests/warn/unused-privates.scala:98:2 ---------
98 |  private object Dongo { def f = this } // warn
   |  ^
   |  unused private member
-- [E198] Unused Symbol Warning: tests/warn/unused-privates.scala:98:17 --------
98 |  private object Dongo { def f = this } // warn
   |                 ^^^^^
   |                 unused private member

Edit: that was due to desugaring to "module val" and "module class".