Remove usage of Dist::Requires.

This was just an experiment to see how Pinto would work if
it used the MYMETA to determine prereqs.  I didn't mean to
commit this (at least not on master).
Pull requires targets.  Add a test for being called w/out any.

Pull requires targets.  Add a test for being called w/out any and
check the exception that gets thrown.
First cut at adding support for pulling packages from cpanfile.

Add an optional argument to the Pull action that names a cpanfile.

1. loads the file using Module::CPANfile,
2. merges prerequisites from the
   phases = qw(configure build test runtime develop) and the
   types = qw(requires recommends suggests)
3. and uses the resulting requirements to add packages to the set
   of targets to be pulled.

Targets are no longer required (in the Moose attribute sense) so
add a test to make sure that we still correctly handle not having
any.