When I designed `mkShell`, I didn't have a good idea of what the output
should look like and so decided to make the build fail. In practice,
this causes quite a bit of confusion and complications because now the
shell cannot be part of a normal package set without failing the CI as
well.
This commit changes that build phase to record all the build inputs in a
file. That way it becomes possible to build it, makes sure that all the
build inputs get built as well, and also can be used as a GC root.
(by applying the same trick as #95536).
The documentation has also been improved to better describe what mkShell
does and how to use it.
The attributes got renamed in PR #126440 and in some places this caused
evaluation errors, e.g. the tarball job was saying (locally)
> attribute 'alsaUtils' missing, at /build/source/nixos/modules/services/audio/alsa.nix:6:4
and I suspect that trunk-combined jobset's failure to evaluate was also caused.
The distinction between the inputs doesn't really make sense in the
mkShell context. Technically speaking, we should be using the
nativeBuildInputs most of the time.
So in order to make this function more beginner-friendly, add "packages"
as an attribute, that maps to nativeBuildInputs.
This commit also updates all the uses in nixpkgs.
In my opinion Functions should only contain pure functions. These are
both meant to provide derivations so I put them under Builders. Don't
know exactly *where* to put them so "special" it is...
