Add a check for `lib` being the first in the pattern

[viraptor]

This is a nixpkgs policy.

Not adding a replacement suggestion at this point, because it seems very tricky to preserve the formatting and potential comments when reordering.

[ilkecan]

I think this should be disabled by default if merged. nixpkgs is a single repository but there are countless other repositories that have Nix code inside them.

There could be a nixpkgs specific flag like --nixpkgs to enable lints such as this.