Add a warning if the file system cache location is relative.

[Carles Pina Estany]

FileBasedCache LOCATION: the documentation says that the path should be absolute, Django accepts absolute or relative.

In ​https://docs.djangoproject.com/en/3.1/topics/cache/#filesystem-caching

It says:
"The directory path should be absolute – that is, it should start at the root of your filesystem."

The code makes them absolute:
​https://github.com/django/django/blob/master/django/core/cache/backends/filebased.py#L22

Enforcing the LOCATION to be specified only in absolute way might have no gain and would have backwards compatibility problems for deployments that are passing the directory using a relative path.

A possible way to improve the situation without or minimal backwards incompatibility changes would be to add a new check (thanks Carlton for the idea).

I'm happy to make any changes:
​https://github.com/django/django/pull/13655

[Mariusz Felisiak <felisiak.mariusz@…>]

In 721c95ba:

Fixed #32180 -- Added system check for file system caches absolute location.