langserver: add godef-based hover backend

[emidoots]

This change adds a godef-based hover backend, similar to the godef-based
textDocument/definition backend added prior. The motivation for this
change is to avoid typechecking the entire program just to serve a single
textDocument/hover request. After this change, hover and definition are
both very quick and use little resources. Larger requests like workspace/symbol,
or textDocument/references, will continue to use more resources as they must
perform typechecking.

The output style of this hover implementation does vary from our prior
typechecking implementation in slight ways, but overall the implementation
always produces results that are on par or slightly better than our typechecking
implementation.

As with the textDocument/definition implementation, we should attempt consolidation
of this hover implementation with our typechecking variant further in the future. At
this point, of course, they are too different to share much code or implementation.

Helps #178
Helps microsoft/vscode-go#853

[keegancsmith]

we have ways to parse that use the virtual filesystem. If you use that you will correctly parse unsaved changes. But it is likely a very minor issue, and I think we can fix all that when we consolidate the implementations.

[emidoots]

Thanks for the comment. This reminded me that I needed to fix hover & jump to definition for unsaved files.

I didn't address your comment directly, though, because as you can see in 8d908eb the fix is not quite as simple. We can't use parseDir directly here because we need it to be aware of the test-suite somewhat. I agree, though, that this is something we can resolve down the line / is very minor.