Invoke create_zipapp.py from the root of the repository and it will
create a minimal zipapp with only the mesonbuild module code and a
__main__.py directly copied from meson.py

The meson.py launcher already tracks the desired entry point, and its
only other effect is to add the mesonbuild directory to the path if it
exists, which it won't in the zipapp. So there's no need to duplicate
this into another __main__.py