Reorganize setup_helpers a bit

Broke apart and reorganized much of `astropy_helpers.setup_helpers`.

Most of the code related to implementing individual distutils commands
has been moved into an astropy_helpers.commands/ subpackage, with a
module devoted to each command.  Some of these modules are very short
and seem a bit silly, but this organization is simply to match the
organization of distutils and setuptools.  It also makes it easy to find
where a given command is implemented.

Also moved most of the functions related to introspecting distutils into
a new distutils_helpers module.

This was part of working on support for custom pre-/post-command hooks, and while it is not *required* for that work I felt like setup_helpers was getting a bit unwieldy and figured I should do something about that before adding more to it.