Closes#496
When running
nix-build ../nixpkgs --arg config.allowUnfree true -A hello-unfree
the package `hello-unfree` is now built rather than getting an
eval-error rejecting instantiation. This is because `config.allowUnfree`
is now interpreted as nested attribute-set declaration, similar to how
it's done in `nix repl`.
To prevent sudden breakage, this behavior was carefully deprecated with
Nix throwing an error if the identifier for `--arg` is not a pure
identifier, but an expression as above.
Any kind of merging is rejected. I.e. doing
nix-build ../nixpkgs --arg config '{cudaSupport = true;}' --arg config.allowUnfree true
is prohibited. That way we don't have to think about merge semantics for
cases like this (or even worse `--arg config 'rec { ... }'`). Another
nice side-effect of this is that we don't need to create an EvalState to
force the values and implement merging.
Change-Id: I8b560883a4468a3f32f915764b08f5fdd8fe71bb
