setup: remove the hack to determine if we can avoid the explicit setuptools-managed...

setup: remove the hack to determine if we can avoid the explicit setuptools-managed dependency on nevow (which was useful for building on dapper)
For simplicity, and to avoid weird failure modes that result from importing nevow during the build process, we now simply require nevow >= 0.6.0.  We currently bundle in misc/dependencies nevow v0.9.18, which will not work on Dapper, since it requires Twisted >= 2.4.0, and Dapper comes with Twisted 2.2.0.  Dapper users can (a) install a newer Twisted, (b) install nevow 0.6.0 in egg form so that setuptools can tell that it is installed (without importing it), (c) beg us to start shipping nevow 0.6.0 instead of nevow 0.9.18 in our bundle.