homeManagerModules attripute naming outside nixosModules

[siryoussef]

adding flake output homeManagerModules.impermanence just not to cause confusion if used with standalone home-manager flake setup (as one usually does not use nixosModules attribute in defining a homeManager module in such a case even though it would work here !, although keeping the old flake output ~nixosModules.home-manager.impermanence is a good idea for backward compatibility.

[SebTM]

Duplicate of https://github.com/nix-community/impermanence/pull/181