Fix completion mode filtering + optimize scopeCompletions (scala#23172)

I've changed scopeCompletions to lazy val to memoize because I beliveve
we don't need to recompute it. The actual fix is at the comment. There
is no need to create a new Completer just to filter all of remaining
completions at later stage.

Fixes scala#23150
[Cherry-picked 1eee888][modified]

Author: tgodzik