Javadoc links support

[Strum355]

Javadoc supports linking to classes/methods etc via {@link ... }. We should also support this in the LSIF output

See: https://docs.oracle.com/en/java/javase/11/docs/specs/doc-comment-spec.html#link

[olafurpg]

Closing this for now since it's a low-priority item and our high-priority goals are ambitious enough. It's worth looking into this again when the sourcegraph backend can consume non-repo sources.