Add overrides for tensorflow-macos

[dbaynard]

This adds the direct override for tensorflow-macos, and adds setuptools as buildInputs for three of its dependencies.
With this change, the only user requirement to get tensorflow-macos working is a workaround for #750.

I was unsure of a few things.

1. This works with buildInputs, so I've used that (rather than, say propagatedBuildInputs) — I presume this is the correct choice?

2. The direct overrides for tensorflow-macos should probably be the same as those for tensorflow itself, but I don't know whether that is desirable.
   If it is not, then the final commit can be dropped.
   (If it is then the latter two commits should probably be merged.)

I've backported the changes from a personal override; I don't really know what the testing policy is, here.

Thanks; happy to contribute this upstream!

[dbaynard]

Ah, just seen (#792) that the way to handle what I do in the first commit is adding entries in build-systems.json. I'll update the PR.