lib/tests/packages-from-directory: make sure all .nix files parse

It is useful that all (or almost all) .nix files in nixpkgs at least
parse since it allows for checking syntax in the repository
programmatically without evaluating anything.

lib/tests/packages-from-directory/c/not-a-namespace/not-a-package.nix

@@ -0,0 +1 @@
+{ }

lib/tests/packages-from-directory/c/support-definitions.nix

@@ -0,0 +1 @@
+{ }