Remove flake-inputs; refactor and split out package

[jurraca]

Besides removing a dependency for the project, this refactors the flake to be a bit cleaner.

Using flake-utils makes expressing "for each system" type expression simple, but it also gets in the way of items that don't need this type of expression, such as the module.nix. Thus we had to define an improper module expression, which makes using the module in another expression more complicated as well.

So instead we create forAllSystems and nixpkgsFor functions explicitly. This means a little more involved package expressions, but that hide less from the user. The module can now be import'ed and it can be used as a module in other machine configurations without extra treatment. I stole this idea from this.

I also took the opportunity to split out the main package expression into its own file, for clarity.

[jurraca]

@virtu would you be willing to take a look? this changes the original module work you did.

[jurraca]

rebased. Made a nix folder for tidiness, and fixed a few things.
This change isn't too risky imo, and I've tested it for Linux.
For other arch's, it would be good to test:

• run nix develop then python check.py and make sure all deps have the green checkmark.
• run nix build then ./result/bin/kartograf and make sure it prints the help message

I have to think if it's worth it to add testing for the module, but for now I've tested it myself (the module issues was the reason for this change).