refactor: reuse disko inside disko-install

[midischwarz12]

Description

Refactor the disko-install cli tool so that it uses the disko cli tool within it.

Motivation

Currently, much of the logic in disko and disko-install, and in cli.nix and install-cli.nix is shared and/or duplicate. In the spirit of code reuse, I think much of this should be refactored. Any additional features for the disko cli would likely have to implemented again for disko-install. Also, there could be unforseen bugs since they have diverged with very different implementations. It's usually a best practice to reuse code when possible because of this.

Proposal

Move much of the code from disko-install and install-cli.nix over to disko and cli.nix. And after, have disko-install run disko from within it. Keep everything beyond just disko to install and such within disko-install. Anything that is required for nixos-install such as mounting with filesystem options and boot options then get separated into a separate install.nix.

[midischwarz12]

I think issues like #1074 make this issue more important because it probably could have been avoided in the case that disko was reused in disko-install. My reasoning is that because flakes are already so established in disko-install, the issue would have never been seen in disko. Plus, disko-install would have to use flakes in disko anyways just by how each are implemented.

[midischwarz12]

Another reason is that tests for disko-install would probably also suffice to test disko as well.