DM-26010: Move mypy execution to root directory, and run it on a directory.

[TallJimbo]

This is the more common and better-supported way to run mypy, but it was previously blocked by the fact that our root (lsst) package was a namespace package. This change should make no external difference in what mypy does right now, but I suspect it will be easier to maintain and I can already say it's much more compatible with at least some in-editor mypy integration tooling.

[timj]

Thanks for this. Setuptools requiring the __init__.py was the final straw. I do wonder whether we are simply misunderstanding namespace packages or too many tools don't yet know what to do with them.

[TallJimbo]

I do wonder whether we are simply misunderstanding namespace packages or too many tools don't yet know what to do with them.

My sense is that they are just a flawed bit of functionality - a good idea whose implementation just leaves it too hard for tools to identify when a directory is a Python package and when it isn't.