Show explain specs in install order

This will match the order that `gem install` would print.
Switch InstallCommand to use RequestSet to install

Gem::DependencyInstaller is still used to set up the RequestSet but the
installation actions occur via RequestSet#install.  This makes --explain
and gem installation use the same gem installation order.

This also moves documentation generation into RequestSet#install.

This makes explain order and install order the same as part of #813