This reverts commit ddfbb4f5.

revert to using a single way for finding the libdir, as my assessment of the OSX bug turned out to be wrong

* the original namespace-collision checker only worked for abstractions
* trying a new namespace-collision checker revealed about 80 new clashes.
  Since there are so many it's probably too disruptive to remove them.
* probably need to revert the namespace clash abstraction removals