lib/modules: Improve error when a configuration is imported

This is appears to be a fairly common mistake for beginners who want
to build larger things from the system configurations, such as NixOps
networks, etc. Further explanation seems appropriate.

lib/modules.nix

@@ -347,6 +347,7 @@ let
       };
 
       result = withWarnings {
+        _type = "configuration";
         options = checked options;
         config = checked (removeAttrs config [ "_module" ]);
         _module = checked (config._module);

lib/tests/modules.sh

@@ -368,6 +368,7 @@ checkConfigError 'The module foo.nix#darwinModules.default was imported into nix
 # _type check
 checkConfigError 'Could not load a value as a module, because it is of type "flake", in file .*/module-imports-_type-check.nix' config.ok.config ./module-imports-_type-check.nix
 checkConfigOutput '^true$' "$@" config.enable ./declare-enable.nix ./define-enable-with-top-level-mkIf.nix
+checkConfigError 'Could not load a value as a module, because it is of type "configuration", in file .*/import-configuration.nix' config ./import-configuration.nix
 
 # doRename works when `warnings` does not exist.
 checkConfigOutput '^1234$' config.c.d.e ./doRename-basic.nix

lib/tests/modules/import-configuration.nix

@@ -0,0 +1,12 @@
+{ lib, ... }:
+let
+  myconf = lib.evalModules { modules = [ { } ]; };
+in
+{
+  imports = [
+    # We can't do this. A configuration is not equal to its set of a modules.
+    # Equating those would lead to a mess, as specialArgs, anonymous modules
+    # that can't be deduplicated, and possibly more come into play.
+    myconf
+  ];
+}