Split Python OBJ importer and exporter, enabling only the importer.

This is from patch D13988. It removes the "- New" from the menu of the
new obj exporter, changes the default addon to just io_import_obj,
and does the right versioning thing.
Also disables the python tests for the old python exporter.

source/blender/blenkernel/intern/blendfile.c

@@ -603,12 +603,12 @@ UserDef *BKE_blendfile_userdef_from_defaults(void)
     const char *addons[] = {
         "io_anim_bvh",
         "io_curve_svg",
+        "io_import_obj",
         "io_mesh_ply",
         "io_mesh_stl",
         "io_mesh_uv_layout",
         "io_scene_fbx",
         "io_scene_gltf2",
-        "io_scene_obj",
         "io_scene_x3d",
         "cycles",
         "pose_library",

source/blender/blenloader/intern/versioning_userdef.c

@@ -973,6 +973,13 @@ void blo_do_versions_userdef(UserDef *userdef)
    */
   {
     /* Keep this block, even when empty. */
+    if (!USER_VERSION_ATLEAST(301, 7)) {
+      /* io_scene_obj directory is gone, split into io_import_obj and io_export_obj,
+       * with io_import_obj enabled by default and io_export_obj replaced by the C++ version.
+       */
+      BKE_addon_remove_safe(&userdef->addons, "io_scene_obj");
+      BKE_addon_ensure(&userdef->addons, "io_import_obj");
+    }
   }
 
   LISTBASE_FOREACH (bTheme *, btheme, &userdef->themes) {