(#7656) Port PMT generator templates into core

Before this patch, the Puppet Module Tool (PMT) generator templates
lived in the PMT codebase -- in a separate project.

This patch ports the Puppet Module Tool (PMT) generator templates into
core Puppet with the following major changes and refactorings:

The PMT generator templates now live under the
lib/puppet/module_tool/skeleton/templates directory. This is a
change from the previous location as the template directory lived at
the root of the PMT project. This change is being made to remain
consistent with the current core Puppet codebase and project layout.

The ported version of lib/puppet/module_tool/skeleton.rb has been
updated to reference generator templates in the
lib/puppet/module_tool/skeleton/templates/ directory.