spell checker:
- find the "closest" symbol for the minimum levenshtein distance
- do not report private symbols from import
- remove spurious error messages on private symbols found by the spell checker
- allow ambiguous symbols to be reported
Merge pull request #4143 from rainers/issue13736

Fix 13736 - Spellchecker should prefer symbols from inner scopes
fix Issue 13757 - [CTFE] extern(C) alias declaration does not work in CTFE