Add support for go-to declaration

[DonJayamanne]

https://code.visualstudio.com/updates/v1_30#_go-to-declaration

[jakebailey]

Since Python doesn't really have "declarations", per se, what we're going to do instead is take the old goto-def behavior (for example, stopping at an import), and make that our "declaration", then make goto-def behave as requested in #1138 (actually going all the way to the real definition).