Fix completion mode filtering + optimize scopeCompletions

[rochala]

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 #23150

[kasiaMarek]

The fix itself seems fine, though there are a lot of unrelated changes here, so it's hard to know what exactly I'm looking at.

[rochala]

Actually, the only unrelated change here is the change of { into fewer braces. The rest was necessary to handle all the test cases.

[tgodzik]

LGTM