pkgs/top-level: make package sets composable

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.

Supersedes #136549
Resolves #114510
Resolves #212494
Resolves #281596

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