evalConfig: lazy unchecked eval of nodes

[fooker]

The nodes, as exposed through the modules extraArgs are lazy evaluated
by not checking the config. This allows to access other machines
configuration without fully evaluation all configurations an thus
avoiding infinite recursion.

[srhb]

While I like the general idea and have wished for it several times, I'm a little worried about how big a footgun it might be in reality, so some questions for clarification:

I think that the "other node" (eg. nodes."db01.example.com" as seen from the perspective of web01.example.com in example/simple.nix) is still checked on its own even if we only build web01-- is that correct? What I want to avoid is accidentally referring to a definition that doesn't have a declaration simply because I'm viewing it from an "unchecked perspective"

[fooker]

As far as I understand, every nodes config is always evaluated and checked (even if filtered). Speaking in the example above: If one has an invalid option in db01.example.com while calling morph with --on=web01.example.com things will still fail.

[srhb]

Thanks!

I'm going to go ahead and merge this because I think it's a good idea (and nicer than some alternatives, like a specific ad-hoc "network" module a la NixOS' lib option). I've smoke tested only a few deployments without changes so let's go ahead and see how it works out. :)