[FEATURE] Support nix as a package manager

[swz-git]

Packages don't show the number of packages installed by nix.

[grtcdr]

I'm only slightly familiar with how nix structures its packages, but not enough to roll out a solution.

Would you like to have a go at this?

Well, provided the solution only relies on the filesystem structure or a database file, as using shell commands is very costly.