Support resumption of partially successful installations and repair of broken installations by making every `Action` idempotent and comparing the actual state of the system to that in the plan or receipt about nix-installer

[Hoverbear]

Hey!

Yes this is one of the things we are sorta working towards! Several actions are already idempotent and we have #35 noting some thoughts. We also would like to be able to recognize existing known-good nix installs and offer a chance to remove them (mostly) cleanly.

This state divergence is indeed an issue and one we can certainly improve on.

A good first step would be working to make all the base actions idempotent, as from there we should be able to make most of the others idempotent. Contributions towards this goal would be most welcome, otherwise we'll likely get to it ourselves in time.

[Hoverbear]

I am going to close this as it is a duplicate of #35 in many ways, this is still an eventual goal!