[Label] Allow specifying full directories/files as order targets

Hello,

I've been fiddling with pytest-order for a personal project.
I wanted to order tests based on files and/or subdirectories, and I believe that it is not feasible at the moment with pytest-order.

Looking a bit into it, I've observed (correct me if I'm misunderstanding something) that the labels should mostly hint at discrete tests, and never at a bunch of other tests (all tests from a directory, file).

Is it possible to provide a feature such as full directory/file-based ordering ? I felt it should be, since the module already supports specifying multiple tests as before/after ordering labels.

I believe it may relate to #50, as the subject matter might end up cross paths.

Note: I've fiddled a bit with the code to obtain something that looks like it might work, but I might also be unknowingly passing-by some major problematics in doing so. If you're interested nonetheless, I can provide an initial PR to serve as a discussion basis, and hopefully contribute to obtaining said feature.

Cheers !

Review pytest-order's existing label behavior and compare the requested directory/file targets with the related discussion in issue #50. Clarify how full directories and files should be represented and interact with existing before/after ordering; the feature is done when those targets reliably order all tests they contain.