DM-9606: Fix Python package structure

[jonathansick]

The lsst/ base directory was missing.

[parejkoj]

Do we really want any python files in here at all? This isn't a package that will be "python imported" ever.

[jonathansick]

Idea is that lsst.validate.metrics.__version__ might be important as it's the native way of establishing package versions in the Stack. Otherwise, there's no Python.

[parejkoj]

Hmm... We don't have another way to track versions? It'd be a bit confusing to have a totally empty python module structure, and might tempt people to add python code to it.

[jonathansick]

Other than interacting with git repositories on the file system or EUPS, we don't afaik.

[jonathansick]

I could add a README in the Python path reminding contributors not to add code to validate_metrics, and look at validate_base instead.

[parejkoj]

Yes, I think a readme would be appropriate, or a note in the base-level readme.

[jonathansick]

Does the README addition work for your @parejkoj ?