Use Pkg to generate Project.toml and the src entrypoint, and add the
Test package from the stdlib.
Also remove some options to generate that added complexity: force,
temp_dir, and ssh. Trying to create a package at an already-existent path
is now an error no matter what. The ssh keyword to generate has been
moved to Template.
TODO:
- Update the file templates to current standards.
- Clean up the gen_plugin signature and update those docs.
- Pkg.add requirements provided to the template.
- Update interactive_template's requirements gathering to use a loop.
- **Make smaller commits.**
