graph: accept a list of source units as input to graph

[pararthshah]

This change will allow srclib-go to graph multiple packages together. The benefit of this is that we can graph all source units of a repo together, taking advantage of the caching in go/loader and gog packages, to avoid doing duplicate work of loading and parsing the same dependencies multiple times.

TODO: fix the global config setting in graph.go

[beyang]

Otherwise LGTM