Don't delete files that are still accessible from the cache

[ikelos]

This is a naive means to ensure we never delete files that are still valid/exist from the cache. The downside with this is that even if a file isn't in the user's symbols path, it might still be retrieved and used (which would be confusing, so the PR can't be committed in its current form). We'd probably need to filter the returned list of symbols to make sure they're remote or that they're in the user's current symbol directory...

Closes #1255