Push cache backend checks down to backend classes

[Adam Johnson]

Currently, the three system checks for caches are individual functions ​in ​django.core.checks.caches. But two of them relate only to FileBasedCache, yet still run regardless of which backends are used. This structure causes some issues:

1. FileBasedCache and its dependencies are imported even when not used.
2. Some waste in check_cache_location_not_exposed() when no FileBasedCache is configured, which resolves some paths before checking against caches.
3. The code structure is a bit messy with repeated loops and isinstance(cache, FileBasedCache) conditions.

I propose restructuring these checks to live within the cache backend classes in django.core.cache.backends.*, adopting the same pattern used for model and field checks, admin checks, and staticfiles finders. (And template backend checks, as I proposed in #35233.)

This would mean:

1. Adding BaseCache.check() which just does return [] for now.
2. Pushing the existing two checks down to a new FileBasedCache.check() method.
3. Dropping the existing code.
4. Checking tests cover the checks sufficiently and they pass with the new structure.
5. Potentially adding a test to cover a custom cache backend with its own check.

[Mariusz Felisiak]

Agreed, we recently did the same thing with constraint check (0fb104dda287431f5ab74532e45e8471e22b58c8).

[bcail]

@Almaz, are you still working on this ticket?

[Almaz Amanzholuly]

Yes, I am still working on this ticket

[Mariusz Felisiak]

​PR