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.