Implement proper UFCS name look up

In basic, UFCS name search looks for only symbols in module scope.
If the enclosing scopes have local imports, the imported overload-set and selected/renamed symbols should also be considered in searching.

In my opinion, UFCS should not see the locally declared symbols (in statement scope and aggregate member scope). So, currently, this is a final step for the UFCS implementation.

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Fixed bugzilla issue:
[Issue 6185](http://d.puremagic.com/issues/show_bug.cgi?id=6185) - Include non-global functions when resolving UFCS