setup: fix site-dirs to find system installed twisted on mac.

setup: fix site-dirs to find system installed twisted on mac.

zooko helped me unravel a build weirdness today.  somehow the system installed
twisted (/System/Library) was pulling in parts of the other twisted (/Library)
which had been installed by easy_install, and exploding.

getting rid of the latter helped, but it took this change to get the tahoe
build to stop trying to rebuild twisted and instead use the one that was
already installed. c.f. tkt #229