LSIF: Code reorganization

Created by: efritz

Some quality of life improvements within the code:

• organize code by entrypoint (server, worker, shared)
• group cohesive files into directories (xrepo + commit, cache + database + backend)
• disambiguate lsp, lsif, and model names via named imports (e.g. RangeId is defined in more than one of these)

This PR only rearranges code, it does not change any implementation. See commits for detailed steps.