Generate a dist-info folder instead of an egg-info

[silky]

All we really need is a METADATA file, and for the folder to have dashes replaced with underscores.

This may be related to https://github.com/nix-community/poetry2nix/issues/616.

I think this probably shouldn't be merged as-is, because I'm just throwing away the egg part, but @adisbladis if you are pro implementing this at all we could:

1. Introduce a new editablePackagesSources argument to decide whether to use egg or dist-info style,
2. Figure out what information is essential, and add that too,
3. ???

Alternatively, this PR by itself might help people that are looking for dist-info files, which get picked up by libraries such as pkg_resources.get_distribution("your-package").