setup: refactor versions-and-paths and use pkg_resources to find them

setup: refactor versions-and-paths and use pkg_resources to find them
Using pkg_resources is probably better if it works -- zope.interface doesn't have a __version__ attribute that we can query, but pkg_resources knows zope.interface's version number, for one thing.
This code falls back to the old way -- looking at the __version__ attributes and __file__ attributes -- if the pkg_resources way doesn't answer.
Note that this patch also changes the capitalization of "Nevow", "Twisted", and "pyOpenSSL", and the spelling of "allmydata-tahoe".  These changes are not frivolous: they are reflecting the fact that we are naming Python packages (technically called Python "distributions") instead of Python modules (technically and confusingly called Python "packages") here.  The package ("distribution") is named "allmydata-tahoe".  The module ("package") is named "allmydata".