Add shasum calculations to Builder objects

The algorithm includes the patch and erb files on disk
Use the builder's shasum method in a software's shasum calculation
Update shasums from the new algorithm
Expose #downloaded_file in the software DSL

This method also restores the old #project_file method, but it has been
deprecated
Fix runtime_dependency instance variable
Move platform truncation logic into `Omnibus::Package`
Ensure an iteration is set in publishing metadata
Move platform_version and platform_shortname calculations into Package