Cache info about files that are not currently opened?

[tau-dev]

I often find myself wanting to find references from, going to definitions in, or renaming definitions in files which aren't open yet − which ZLS currently cannot do.

It may of course be impractical to keep full information for all files in memory, especially for bigger projects, but I wonder if we could cache just enough information to enable this use-case? Maybe a list of all imports (and possibly all contained identifiers) per file would be sufficient.

This may require some modifications to the resource handling to enable analysis passes to temporarily open files for parsing the complete information.