MAINT: astroquery.mast caches into root cache

I've noticed that a lot of caching directories are created for astroquery.mast, mostly due to the fact that there are multiple classes provided in the module (as opposed to other modules that more strictly following the original astroquery API). I think it would be best to consolidate them into a `mast` base directory as `Observations` or `Catalogs` are not exactly unambiguous.

@jaymedina

No files or tests are named. Start by locating the cache configuration and class entry points under astroquery.mast, including Observations and Catalogs, then trace how their cache directories are chosen. Done means the mast classes consistently cache beneath one mast base directory without breaking existing cache behavior.

Written by the indexing model from the issue text.