flake-info: support importing a single nixpkgs attribute

[raboof]

to make testing somewhat faster

[Jaculabilis]

If this is intended to make testing faster, it might be a good idea to have this force --kind package, since otherwise it will import just the one package attribute... and then thousands of NixOS options, which isn't very fast.

[raboof]

If this is intended to make testing faster, it might be a good idea to have this force --kind package, since otherwise it will import just the one package attribute... and then thousands of NixOS options, which isn't very fast.

Good point: I had that locally at some point but apparently didn't commit it to this branch - was juggling so many unmerged branches :)

Would you be interested in creating a new PR with this change but enhanced to imply --kind package in this case?

[Jaculabilis]

Sure, changes are up at #934

[raboof]

superseded by #934