(PUP-2079) Add no-op PMT generate template format.

Prior to this commit, the mechanisms available for writing skeletons containing ERB templates
were awkward at best (and broken at worst).

This change adds a new template extension for module skeletons that has the (somewhat counter-
intuitive, when described this way) behavior of performing no alterations to the given template.
In this way, one could place a file like `foo_bar.erb.template` in their module skeleton, and
generated modules would contain a file named `foo_bar.erb` with the exact contents of the
`.template` file.

This will also close pull requests #2468 and #2688.