Remove display options (--name, etc.) from the Distribution class.

Remove display options (--name, etc.) from the Distribution class.

These options were used to implement “setup.py --name”,
“setup.py --version”, etc. which are now handled by the pysetup metadata
action or direct parsing of the setup.cfg file.

As a side effect, the Distribution class no longer accepts a 'url' key
in its *attrs* argument: it has to be 'home-page' to be recognized as a
valid metadata field and passed down to the dist.metadata object.

I cleaned up some comments, docstrings and code along the way.