lib/modules: Make sure to not import module _file's into the store

Previously if `_file` was specified by a module:
  trace: warning: The type `types.string' of option `foo' defined in `/nix/store/yxhm2il5yrb92fldgriw0wyqh2kk9qyc-bug.nix' is deprecated. See https://github.com/NixOS/nixpkgs/pull/66346 for better alternative types.

With this change:
  trace: warning: The type `types.string' of option `foo' defined in `/home/infinisil/src/nixpkgs/bug.nix' is deprecated. See https://github.com/NixOS/nixpkgs/pull/66346 for better alternative types.

1 files changed, 2 insertions, 2 deletions

diff --git a/lib/modules.nix b/lib/modules.nix
index 02a669df65938..5a3a268cafa32 100644
--- a/lib/modules.nix
+++ b/lib/modules.nix
@@ -224,7 +224,7 @@ rec {
       if badAttrs != {} then
         throw "Module `${key}' has an unsupported attribute `${head (attrNames badAttrs)}'. This is caused by introducing a top-level `config' or `options' attribute. Add configuration attributes immediately on the top level instead, or move all of them (namely: ${toString (attrNames badAttrs)}) into the explicit `config' attribute."
       else
-        { _file = m._file or file;
+        { _file = toString m._file or file;
           key = toString m.key or key;
           disabledModules = m.disabledModules or [];
           imports = m.imports or [];
@@ -232,7 +232,7 @@ rec {
           config = addFreeformType (addMeta (m.config or {}));
         }
     else
-      { _file = m._file or file;
+      { _file = toString m._file or file;
         key = toString m.key or key;
         disabledModules = m.disabledModules or [];
         imports = m.require or [] ++ m.imports or [];