setup: don't try __import__(name) in _auto_deps.py

setup: don't try __import__(name) in _auto_deps.py
This happens to work, because all of our "distribution" (i.e. distributable packaged Python code) names to coincide with all of their "package" (i.e. a directory with a __init__.py in it, which is "import"-able) names, except, I think for Twisted on Brian's debian sid system.

But there's no reason why it should always work, and the only reason for that __import__() was to give us an explicit error message indicating missing requirements in the case that pkg_resources isn't importable or that the requirements don't have correct .egg-info metadata.  So, by removing this stanza we may allow certain places to get a more ad-hoc failure message, i.e. an ImportError from somewhere, instead of an ImportError from _auto_deps.py, but that's okay.

Note that dependencies which do not have their .egg-info metadata with them are increasingly rare, since Python 2.5 distutils creates the .egg-info file by default, and Linux distributions have stopped their former practice of actively deleting the .egg-info files.