Not show module names

Fixes #633
SyntaxKind.StringLiteral would also include string-keyed property names, which you do want to show in completion (see the block directly below this).

In terms of asking for whether something is an external module, there is only a function isExternalModule, which takes a SourceFile, so that will not help you here. You could write a similar utility for ambient external modules (with quoted names). In order for it to work on a Symbol, you'd want to ask an arbitrary declaration is an external module, based on whether it is a module and its name has quotes. Why is it sufficient to just ask one declaration? Well, external modules can merge, but only with other external modules. So if one is an external module, they are all external modules.