Sharing a first piece of common code between all package sets makes it
easier to maintain and less likely to introduce a new package set
without this.

This adds some basic tests to compose package sets. The cases that are
currently broken
are commented out, they include things like:

- pkgsStatic.pkgsMusl losing the isStatic flag
- pkgsCross.ppc64-musl.pkgsMusl losing the gcc.abi setting
- pkgsCross.mingwW64.pkgsStatic losing the config string
- pkgsLLVM.pkgsMusl losing the useLLVM flag
- pkgsLLVM.pkgsStatic losing the useLLVM flag
- pkgsLLVM.pkgsi686Linux losing the useLLVM flag

And probably more.

The various pkgsXYZ top-level package sets did not pass localSystem /
crossSystem to lower levels, so far. This change propagates original
arguments to lower levels, which include the overrides made by an upper
package sets.

There is an extensive test-suite to test various combinations of package
sets in pkgs/test/top-level. There are a few basic promises made:

- Package sets must be idempotent. pkgsMusl.pkgsMusl === pkgsMusl.

- Once pkgsCross is used any subsequent package sets should affect the
  **host platform** and not the build platform. Examples:
  - pkgsMusl.pkgsCross.aarch64-multiplatform is a cross compilation from
musl to glibc/aarch64
  - pkgsCross.aarch64-multiplatform.pkgsMusl is a cross compilation to
musl/aarch64

- Modifications from an earlier layer should not be lost, unless
  explicitly overwritten. Examples:
  - pkgsStatic.pkgsMusl should still be static.
  - pkgsStatic.pkgsCross.gnu64 should be static, but with glibc instead
of musl.

Exceptions / TODOs:
- pkgsExtraHardening is currently not idempotent, because it applies the
  same flags over and over again.

Resolves NixOS#136549
Resolves NixOS#114510
Resolves NixOS#212494
Resolves NixOS#281596

When using pkgsCross with a system that ends up the same as the
localSystem, then modifications for package sets like pksgMusl need to
be done for **both** localSystem and crossSystem. Consider the following
on x86_64-linux:

  pkgsCross.gnu64.pkgsMusl

Before this change, this would result in a musl buildPlatform, but a gnu
hostPlatform. This breaks the promise of "stacking" package sets on top
of each other.

After this change, it results in a musl buildPlatform and a musl
hostPlatform. This works better.

One could expect this to result in the same as pkgsCross.musl64, i.e. a
gnu buildPlatform and a musl hostPlatform, however I couldn't get this
to work without increasing memory usage for ci/eval by many, many GB.
This is caused by usage of pkgsi686Linux inside the main package set,
which follows the same hybrid pattern.