Improve Hover Response Caching #450
Description
This is already noted in the hover-request.ts file, which should be changed to a separate caching service. This is particularly important since previously computed hover results for libs will stay put even if the libs are no longer reachable from our config.
An example of this is checking the hover result for a lib that can be reached, forcing it to compute & cache, and then checking it again after its corresponding libs entry has been removed from the config. Hovering again will show the diagnostic for the unresolved include along with the cached hover result. We should invalidate those cache entries when the libs change.