Use cross-repo hover message instead of "partial semantic" label
Created by: olafurpg
Fixes #21057 (closed). Previously, Sourcegraph would use search-based code intel to populate the hover message in the following scenario:
- the call-site has LSIF, but no hover message for externally imported symbols
- the definition-site has LSIF, including hover messages for exported symbols (which are imported from call-site)
With this commit, Sourcegraph queries the LSIF store to find the hover message that's attached to the definition location of the moniker.
cc/ @sourcegraph/code-intel