ansible_mitogen: Speedup startup by not sending `__main__`

On my laptop his reduces the time to execute `ansible -mping ...` by approx
300 ms with `strategy=mitogen_linear`.

Until this commit Mitogen was unnecessarily sending large chunks of Ansible
from the controller to targets, due to `__main__` being identified as a
related module of `ansible.module_utils.basic`, and resolving to something
within `ansible.cli...`.

On Ansible target hosts executing any Ansible Module `__main__` is imported by
`ansible.module_utils.basic` as part of Ansible's module delivery mechanism.
When `mitogen.master.ModuleResponder` (on the controller) processes the
request for `ansible.module_utils.basic` from the target, it scans
`ansible.module_utils.basic` for related imports and finds `__main__`. However
`__main__` on the controller is not the same module as `__main__` on the
target. On the controller it is a module in `ansible.cli...` that implements
one of the ansible commands (e.g. `ansible`, `ansible-playbook`).

docs/changelog.rst

@@ -29,6 +29,8 @@ In progress (unreleased)
 * :gh:issue:`1118` CI: Add OS release coverage: Ubuntu 22.04, Ubuntu 24.04
 * :gh:issue:`1124` :mod:`mitogen`: Log why a module is sent or not sent by
   :class:`mitogen.master.ModuleResponder`
+* :gh:issue:`1124` :mod:`ansible_mitogen`: Speedup startup by not sending
+  ``__main__`` as a related module
 
 
 v0.3.33 (2025-11-22)

mitogen/master.py

@@ -843,6 +843,12 @@ class ModuleFinder(object):
     related modules likely needed by a child context requesting the original
     module.
     """
+
+    # Fullnames of modules that should not be sent as a related module
+    _related_modules_denylist = frozenset({
+        '__main__',
+    })
+
     def __init__(self):
         #: Import machinery is expensive, keep :py:meth`:get_module_source`
         #: results around.
@@ -954,6 +960,9 @@ class ModuleFinder(object):
         if 'six.moves' in related_fullname:
             return _log_reject('six.moves avoidence')
 
+        if related_fullname in self._related_modules_denylist:
+            return _log_reject('on denylist')
+
         return False
 
     def find_related_imports(self, fullname):