rename Berkshelf::CookbookSource to Berkshelf::Dependency
update references of source/sources to dependency/dependencies in Downloader
rename source/sources to dependency/dependencies in Berksfile
rename source/source to dependency/dependencies in Resolver

rename add_source_dependencies to add_recursive_dependencies
rename source/sources in Lockfile while preserving the structure of a

lockfile. The lockfile still contains an entry called "sources". We will need
to migrate old lockfiles again to update these lockfiles to the proper terminology
fix error message expectations in features and clarify them a bit