Add RequestSet#pretty_print

This has a subset of the data needed to diagnose problems with
RequestSet.  More data may be required in the future.
Maintain self-dependency for gemspec

If the gemspec doesn't create a dependency for itself it won't be
included in the lockfile and won't be installed on the next go-around.

Part of #1020
Add gemspec and deps to lockfile DEPENDENCIES list

Previously the dependencies for the gemspec itself and its dependencies
were not added to the lockfile.  This may have caused an
incompatibility, but it is unknown if it would be automatically fixed
by bundler or not, nor if it caused any real damage.

Fixes #1020