lib.modules: Use types.optionType for _module.freeformType

This ensures that the module file locations are propagated to the
freeform type, which makes it so that submodules in freeform types now
have their declaration location shown in the manual, fixing
https://github.com/NixOS/nixpkgs/issues/132085.

In addition, this also newly allows freeformTypes to be declared
multiple times and all declarations being merged together according to
normal option merging.

This also removes some awkwardness regarding the type of `freeformType`

lib/tests/modules/freeform-submodules.nix

@@ -0,0 +1,22 @@
+{ lib, options, ... }: with lib.types; {
+
+  options.fooDeclarations = lib.mkOption {
+    default = (options.free.type.getSubOptions [])._freeformOptions.foo.declarations;
+  };
+
+  options.free = lib.mkOption {
+    type = submodule {
+      config._module.freeformType = lib.mkMerge [
+        (attrsOf (submodule {
+          options.foo = lib.mkOption {};
+        }))
+        (attrsOf (submodule {
+          options.bar = lib.mkOption {};
+        }))
+      ];
+    };
+  };
+
+  config.free.xxx.foo = 10;
+  config.free.yyy.bar = 10;
+}