Vendor most packager-specific generator files
Use vendored assets in the generation steps
Do not generate pkg, dmg, or msi assets by default, expose CLI option
Fix broken references and cleanup generator
Remove Project#files_path and just use Project#resources_path
Default Project#resources_path to `root/resources/NAME`
Do not purge or create unnecessary staging directories
Add an `id` method to all packagers for their namespaces
Raise a NotImplementedError when calling package_name in Base
Simplify logic for resource searching

This commit makes it easy for Omnibus to search for a resource (asset)
by first looking in the expected local directory as defined by
Project#resources_path, and then defaulting back to the file vendored
within Omnibus.
Use the new resource_path method in the deb packager
Use the new resource_path method in the rpm packager
Use the new resource_path method in the dmg packager
Build is no longer a required stage
Refactor MSI packager to use templates and new hottness
Refactor PKG packager to use templates and new hottness
Remove unused Forwardable module
Move package_name higher in the class for readability
Make all packager methods public
Various visual cleanup in Packager::Base