[3006.x] Constrain the localfs cache to the cachedir

[lkubb]

What does this PR do?

Constrain the localfs cache module to its cachedir.

What issues does this PR fix or reference?

Fixes: https://github.com/saltstack/salt/issues/67031

Previous Behavior

Operates on the whole filesystem when bank (or key) is prefixed with os.sep

New Behavior

Only operates on subpaths of cachedir.

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [x] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [x] Tests written/updated

Commits signed with GPG?

Yes

[twangboy]

Once we get the test suite fixed we'll need you to rebase so the tests can run.