hghave: add check for bdist_mpkg stable

hghave: add check for bdist_mpkg

--- a/tests/hghave.py	Wed Apr 27 22:45:52 2016 -0400
+++ b/tests/hghave.py	Mon Apr 18 23:55:58 2016 -0400
@@ -456,6 +456,14 @@
 def has_osx():
     return sys.platform == 'darwin'
 
+@check("bdistmpkg", "bdist_mpkg Python Package")
+def has_bdistmpkg():
+    try:
+        import bdist_mpkg.script_bdist_mpkg
+        return True
+    except ImportError:
+        return False
+
 @check("docker", "docker support")
 def has_docker():
     pat = r'A self-sufficient runtime for'