Many attributes can take moduleArgs when auto-loaded in order to
facilitate access to them from other files. Those same attributes could
not take moduleArgs when included directly, which was inconsistent.
With this change, all attributes that could take moduleArgs when
auto-loaded can now always do so. Auto-loading no longer needs special
cases.
Previously, the packages outputs would be evalutated using callPackage
on the package set, but the package set already contains the package. If
another package depends on it, the dependency would be the version from
the package set. By grabbing the packages from the package set, each
package only needs to be evaluated once.
Since evaluating pkgs is expensive, this speeds up evaluation of flakes
with `packages.default` by attempting to get the name using a pkgs set
containing only mocked builders that just return the name.
In order to speed up evalutation of flakes setting `packages.default`,
this trys to compute the pname in multiple ways, ordered by speed. The
pname can be set manually, which is fastest. To avoid computing a new
nixpkgs and flakelight overlays, the second option just uses callPackage
from stock nixpkgs. If that fails, the prior, most accurate, method is
used which creates a new nixpkgs with all the overlays available.
Using `perSystem` to implement per-system attributes ties all the
per-system attributes together; all the `perSystem` functions must run
to determine output attrs. By generating them separately, the generation
can be done lazily.
