Support pip --no-deps in spec files

[jchorl]

pip --no-deps seems to be a supported third-party package manager. However, with a spec file, there's no way to specify it. The rest of the plumbing is already in-place.

Quick tweak to support specifying it in spec files.

Testing is tough because the response doesn't contain the third-party packages to be installed. I did check though that pip was accurately picked up.

[AntoinePrv]

I don't think this is a super nice option, I'd prefer having an option like pip: args: ["--no-deps"].
personally I also don't think we should absorb all the legacy of conda, but try to improve where it makes sense.