osx: use bdist_mpkg.script_bdist_mpkg module instead of bdist_mpkg command

It seems like a default installation of bdist_mpkg makes it available as
Python module, but the corresponding executable is placed in a location like
/System/Library/Frameworks/Python.framework/Versions/2.7/Extras/bin which is
not in $PATH and thus not directly available. 'make osx' would thus fail.

Instead, skip the bdist_mpkg executable and invoke it as a Python module. That
works out of the box here.

Makefile

@@ -136,10 +136,10 b' i18n/hg.pot: $(PYFILES) $(DOCFILES) i18n'
 # Packaging targets
 
 osx:
-	@which bdist_mpkg >/dev/null || \
+	python -c 'import bdist_mpkg.script_bdist_mpkg' || \
 	   (echo "Missing bdist_mpkg (easy_install bdist_mpkg)"; false)
 	rm -rf dist/mercurial-*.mpkg
-	bdist_mpkg setup.py --
+	python -m bdist_mpkg.script_bdist_mpkg setup.py --
 	mkdir -p packages/osx
 	N=`cd dist && echo mercurial-*.mpkg | sed 's,\.mpkg$$,,'` && hdiutil create -srcfolder dist/$$N.mpkg/ -scrub -volname "$$N" -ov packages/osx/$$N.dmg
 	rm -rf dist/mercurial-*.mpkg