Imported modules reported as both unknown and known in dict literal in diagnostic

[jakebailey]

Forcing Analyzer.EnableDiagnostics to test linting revealed that imported modules are reported as both unknown and known (with info) when used in a dict literal (both as a key and as a value).

Part of #242.

[jakebailey]

Commenting out this bit (at least the warn) seems to help (and in other cases too):

https://github.com/Microsoft/python-language-server/blob/ea8de675088804963cfbbe67948e39510267a251/src/Analysis/Engine/Impl/Analyzer/ExpressionEvaluator.cs#L156-L160

My test case is ptvsd.wrapper, where commenting out the above reduces the number of warnings by a whopping 171.