lib/systems: elaborate properly with non-matching system / config / parsed args

When elaborating a system with both "config" and "system" arguments
given, they might not match the parsed results.  Example:

elaborate {
  config = "i686-unknown-linux-gnu";
  system = "x86_64-linux";
}

This would result in a parsed system for i686, because the config
argument is preferred.  But since "// args //" comes after system has
been inferred from parsed, it is overwritten again.  This results in
config and parsed all pointing to i686, while system still tells the
story of x86_64.

Inconsistent arguments can also be given when passing "parsed" directly.
This happened in stage.nix for the various package sets.

The solution is simple: One of the three arguments needs to be treated
as the ultimate source of truth.  "system" can already be losslessly
extracted from "parsed".  However, "config" currently can not, for
example for various -mingw32 cases.  Thus everything must be derived
from "config".

To do so, "system" and "parsed" arguments are made non-overrideable for
systems.elaborate.  This means, that "system" will be used to parse when
"config" is not given - and "parsed" will be ignored entirely.

The systemToAttrs helper is exposed on lib.systems, because it's useful
to deal with top-level localSystem / crossSystem arguments elsewhere.

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