…problems

This makes it such that these two errors can both be thrown for a single
package:
- The attribute value not being a derivation
- The attribute not being a proper callPackage

The tests had to be adjusted to only throw the error they were testing
for

Convenience function to run another validation over a successful validation result.

This will be usable in more locations in future commits, making the code
nicer.

This prepares the code base for the removal of the `--version` flag, to
be replaced with a flag that can specify a base version to compare the
main Nixpkgs against, in order to have gradual transitions to stricter
checks.

This refactoring does:
- Introduce the `version` module that can house the logic to increase
  strictness, with a `version::Nixpkgs` struct that contains the
  strictness conformity of a single Nixpkgs version
- Make the check return `version::Nixpkgs`
- Handle the behavior of the still-existing `--version` flag with `version::Nixpkgs`
- Introduce an intermediate `process` function to handle the top-level
  logic, especially useful in the next commit

This implements the option for a gradual migration to stricter checks.
For now this is only done for the check against empty non-auto-called
callPackage arguments, but in the future this can be used to ensure all
new packages make use of `pkgs/by-name`.

This is implemented by adding a `--base <BASE_NIXPKGS>` flag, which then
compares the base nixpkgs against the main nixpkgs version, making sure
that there are no regressions.

The `--version` flag is removed. While it was implemented, it was never
used in CI, so this is fine.

This implements the ability to test gradual transitions in check
strictness, and adds one such test for the empty non-auto-called
arguments check.

Previously, not passing `--base` would enforce the most strict checks.
While there's currently no actual violation of these stricter checks,
this does not match the previous behavior.

This won't matter once CI passes `--base`, the code handling the
optionality can be removed then.

This would be duplicated otherwise