Rename `inputs.nixpkgs` to `inputs.dev-nixpkgs` to avoid shadowing the
root flake's `inputs.nixpkgs`.
Having the dev flake shadow an input can be confusing, since overriding
the root flake's input will have no effect when evaluating outputs that
depend on the dev partition.
The only reason we specify a `nixpkgs` input at all here, is so the
other inputs can follow it.
Once nix 2.26 is more prevalent, we can follow the root flake's inputs
using a `"path:../.."` input.
This removes the need for end-users to manually set
`nixvim.inputs.devshell.follows = ""` (etc)
We offload evaluation of some of our flake modules into a `dev`
partition submodule.
- When its not needed, this submodule is not evaluated.
- When it is needed, it fetches extra inputs from `flake/dev/flake.nix`
as part of evaluating the submodule.
See https://flake.parts/options/flake-parts-partitions.html
