Make Go indexer a rolemodel for indexer implementations

[emidoots]

Features:

• [x] Method documentation
• [x] const/var documentation
• [ ] Emit repo/file/line/column info for later use cases (e.g. jump to view source on Sourcegraph / code host)

Bugs:

• [x] Viewing root (top level) Go packages doesn't work ("." recorded as path by indexer?)
• [ ] "Package graphqlbackend" should be accompanied by it's description (not full detail) in schema.
• [ ] Implement the full schema
• [ ] Eliminate all TODOs
• [ ] Add tests (use autogold snapshotting)
• [ ] Some functions signatures not complete/working at e.g. http://localhost:3333/github.com/sourcegraph/sourcegraph/-/go/-/cmd/frontend/graphqlbackend
• [ ] Should track packages by path+pkgName, not pkgName (e.g. multiple main packages are "merged")
• [ ] func _() appears in http://localhost:3333/github.com/sourcegraph/sourcegraph/-/go/-/cmd/searcher/protocol