User argv[0] to internally relaunch meson.py

When installing Meson distutils may choose to put shim scripts in the
PATH that only set up the egg requirements before launching the real
`meson.py` contained in the egg.

This means that __file__ points to the real `meson.py` file, but
launching it directly is doomed to fail as it's missing the metadata
contained in the shim to set up the path egg, resulting in errors when
trying to import the `mesonbuild` module.

A similar issue affects Meson when installed as a zipapp, with the
current code going great lengths to figure out how to relaunch itself.

Using argv[0] avoids these issues as it gives us the way the current
executable has been launched, so we are pretty much guaranteed that
using it will create another instance of the same executable.

meson.py

@@ -17,14 +17,4 @@
 from mesonbuild import mesonmain
 import sys, os
 
-def main():
-    thisfile = __file__
-    if not os.path.isabs(thisfile):
-        thisfile = os.path.normpath(os.path.join(os.getcwd(), thisfile))
-    if __package__ == '':
-        thisfile = os.path.dirname(thisfile)
-
-    sys.exit(mesonmain.run(thisfile, sys.argv[1:]))
-
-if __name__ == '__main__':
-    main()
+sys.exit(mesonmain.run(sys.argv[0], sys.argv[1:]))