Move more pin-caching logic from DocMap to PinCache

[apiology]

Changes in logic:

• We treat the parser gem as a special case given its degenerate behavior with YARD, and only ingest RBS pins. https://github.com/castwide/solargraph/pull/1005 provides enough RBS updates to be able to typecheck solargraph in strong mode, which ought to be enough for our use. If YARD performance becomes enough of an issue maybe this could be a configuration item someday.
• More nuanced logging for what type of RBS collection is being used (local shims, gem-provided types, or central RBS collection)

[apiology]

This is another significant chunk out of #1006. There's a bit of overlap with #1062 and https://github.com/castwide/solargraph/pull/1061 where necessary, but it should be straightforward enough to resolve any conflicts after the first is merged.