Emit type-definitions data

[alidn]

Adds support for Goto Type Definition Request, a prerequisite for sourcegraph/sourcegraph#13605

Checklist

• [x] Emit type-definition data for all symbols that don't have a primitive type (like structs, interfaces, some pointers and slices)
• [x] Add tests for symbols and types defined in the same package
• [x] Add tests for symbols and type defined in different packages
• [ ] Submit a pull-request to sourcegraph/lsif-protocol to add support for type defs (it currently uses alidn/lsif-protocol)
• [ ] A clean-up and update documentations

And maybe that's it?