stdenv: separate all meta-checking code (~200 lines)

Only cosmetic changes are done otherwise.
Real refactoring is left for later.

There's a small slow-down on my machine:
$ time nix-env -qa -P >/dev/null
gets from ~2.8 to ~3.5 seconds (negligible change in RAM).
That's most likely caused by sharing less computation between different
mkDerivation calls, and I plan to improve that soon.