Fix and improve types for flake outputs

[figsoda]

nil was giving me questionable completions - packages.default is not supposed to be a derivation, packages.${system}.default is. This PR fixes this issue for packages and devShells, and also fixes and improves types for nixosConfigurations. I wasn't able to get completion working correctly, but that seems to be a separate issue since I also couldn't get inputs.*.inputs.* to complete follows

[oxalica]

I wasn't able to get completion working correctly, but that seems to be a separate issue since I also couldn't get inputs.*.inputs.* to complete follows

Because the rest type { _: ty } is not implemented yet, only types of constant keys are.

nil/crates/ide/src/ty/mod.rs

Lines 31 to 37 in 6852252

	({ $($key:literal : $ty:tt),* $(,)? $(_ : $rest_ty:tt)? }) => {{
	// TODO: Rest type.
	$(let _ = ty!($rest_ty);)?
	$crate::ty::Ty::Attrset($crate::ty::Attrset::from_internal([
	$(($key, ty!($ty), $crate::ty::AttrSource::Unknown),)*
	]))
	}};

Nevertheless, this patch is good to have.