Support PEP-639 style License-Expression fields

[peterebden]

These are superseding the old freeform text License: fields. There are some packages in the wild that have this and not Licence any more, presumably that will increase over time.

https://peps.python.org/pep-0639/

These are SPDX expressions. Please doesn't parse those for now, but it can crudely support them by simply entering each expression as a licence. In the future when we add the ability to parse them, this should just work.

Also fixed a couple of existing bugs about how the repo argument got used in pip_library.

[chrisnovakovic]

Perhaps we should generate this wheel at test time, just to prove there's nothing weird going on in this plugin (a la CVE-2024-3094)? It'd be pretty straightforward too, given that we could build it with arcat. If you want to save keystrokes and a bit of complexity, fakepkg/__init__.py isn't necessary - a wheel consisting only of metadata is legit (and occasionally seen in the wild as a way of implementing virtual packages).