Revisit the need for and usage of FileContainer

As discussed in #1791, there is a need to:

* Revisit the necessity of `FileContainer`. Is there a reason to not just use built-in `astropy.utils.data` functions directly in the query modules?
* Why is the usage of `FileContainer` inconsistent between the different modules in the package?

Start by reading issue #1791 and locating FileContainer usage across the query modules. Compare that usage with the built-in astropy.utils.data functions and document why the implementations differ. Done means reaching and recording a project-wide decision on whether FileContainer remains necessary and how its usage should be made consistent.

Written by the indexing model from the issue text.