Ensure builtin module registration is deterministic

src/ocrmypdf/_plugin_manager.py

@@ -67,7 +67,9 @@ class OcrmypdfPluginManager(pluggy.PluginManager):
 
         # 1. Register builtins
         if self.__builtins:
-            for module in pkgutil.iter_modules(ocrmypdf.builtin_plugins.__path__):
+            for module in sorted(
+                pkgutil.iter_modules(ocrmypdf.builtin_plugins.__path__)
+            ):
                 name = f'ocrmypdf.builtin_plugins.{module.name}'
                 module = importlib.import_module(name)
                 self.register(module)

src/ocrmypdf/builtin_plugins/__init__.py

@@ -3,3 +3,7 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# This file exists only mark builtin_plugins as a package.
+# The plugin manager will not load it, so anything defined here may not be
+# processed as a module.