Add script to create a zipapp.

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

packaging/create_zipapp.py

@@ -0,0 +1,22 @@
+#!/usr/bin/env python3
+
+import argparse
+from pathlib import Path
+import shutil
+import sys
+import tempfile
+import zipapp
+
+parser = argparse.ArgumentParser()
+parser.add_argument('source', nargs='?', default='.', help='Source directory')
+parser.add_argument('--outfile', default='meson.pyz', help='Output file for the zipapp')
+parser.add_argument('--interpreter', default='/usr/bin/env python3', help='The name of the Python interpreter to use')
+
+options = parser.parse_args(sys.argv[1:])
+
+source = Path(options.source).resolve()
+
+with tempfile.TemporaryDirectory() as d:
+    shutil.copy2(source / 'meson.py', Path(d, '__main__.py'))
+    shutil.copytree(source / 'mesonbuild', Path(d, 'mesonbuild'))
+    zipapp.create_archive(d, interpreter=options.interpreter, target=options.outfile)

run_meson_command_tests.py

@@ -193,9 +193,10 @@ def test_meson_exe_windows(self):
     def test_meson_zipapp(self):
         if is_windows():
             raise unittest.SkipTest('NOT IMPLEMENTED')
-        source = Path(__file__).resolve().parent.as_posix()
+        source = Path(__file__).resolve().parent
         target = self.tmpdir / 'meson.pyz'
-        zipapp.create_archive(source=source, target=target, interpreter=python_command[0], main=None)
+        script = source / 'packaging' / 'create_zipapp.py'
+        self._run([script.as_posix(), source, '--outfile', target, '--interpreter', python_command[0]])
         self._run([target.as_posix(), '--help'])