Reintroduce Python Zipapp support

* Added `main` function as an entry point.
 * Created `__main__.py` to allow the creation of Zipapps without explicitly
   calling `meson:main`.

__main__.py

@@ -0,0 +1,20 @@
+#!/usr/bin/env python3
+
+# Copyright 2016 The Meson development team
+
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+
+#     http://www.apache.org/licenses/LICENSE-2.0
+
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import meson
+import sys
+
+sys.exit(meson.main())

meson.py

@@ -15,11 +15,14 @@
 # limitations under the License.
 
 from mesonbuild import mesonmain
-import sys, os, os.path
+import sys, os
 
+def main():
     launcher = sys.argv[0]
     # resolve the command path if not launched from $PATH
     if os.path.split(launcher)[0]:
         launcher = os.path.realpath(launcher)
+    return mesonmain.run(launcher, sys.argv[1:])
 
-sys.exit(mesonmain.run(launcher, sys.argv[1:]))
+if __name__ == '__main__':
+    sys.exit(main())