Revert "lib/types: Allow paths as submodule values"

This reverts commit eec83d4.

This broke hydra evaluation because with this commit submodule values
are allowed to be paths, however the certmgr module uses `either
(submodule ...) path` in its type, meaning it already used paths for
something else which would now be interpreted as a submodule.

lib/types.nix

@@ -376,16 +376,14 @@ rec {
           else unify (if shorthandOnlyDefinesConfig then { config = value; } else value);
 
         allModules = defs: modules ++ imap1 (n: { value, file }:
-          if isAttrs value || isFunction value then
-            # Annotate the value with the location of its definition for better error messages
-            coerce (lib.modules.unifyModuleSyntax file "${toString file}-${toString n}") value
-          else value
+          # Annotate the value with the location of its definition for better error messages
+          coerce (lib.modules.unifyModuleSyntax file "${toString file}-${toString n}") value
         ) defs;
 
       in
       mkOptionType rec {
         name = "submodule";
-        check = x: isAttrs x || isFunction x || path.check x;
+        check = x: isAttrs x || isFunction x;
         merge = loc: defs:
           (evalModules {
             modules = allModules defs;

nixos/doc/manual/development/option-types.xml

@@ -257,9 +257,9 @@
     <listitem>
      <para>
       A set of sub options <replaceable>o</replaceable>.
-      <replaceable>o</replaceable> can be an attribute set, a function
-      returning an attribute set, or a path to a file containing such a value. Submodules are used in
-      composed types to create modular options. This is equivalent to
+      <replaceable>o</replaceable> can be an attribute set or a function
+      returning an attribute set. Submodules are used in composed types to
+      create modular options. This is equivalent to
       <literal>types.submoduleWith { modules = toList o; shorthandOnlyDefinesConfig = true; }</literal>.
       Submodules are detailed in
       <xref