Make Go indexer a rolemodel for indexer implementations

[emidoots]

Features:

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

Bugs:

• 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