[PackageLoading] Introduce ManifestLoaderProtocol.
[PackageLoading] Introduce ManifestLoaderProtocol.

 - This is intended to allow clients to be decoupled from the actual
   implementation, currently just so that it can be mocked for tests that
   require Manifests but don't need to go through the full manifest loading
   process.

abertelrud/swift-package-manager,apple/swift-package-manager,abertelrud/swift-package-manager,mxcl/swift-package-manager,sschiau/swift-package-manager,abertelrud/swift-package-manager,sschiau/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,mxcl/swift-package-manager,vmanot/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,JGiola/swift-package-manager,abertelrud/swift-package-manager,haskellswift/swift-package-manager,JGiola/swift-package-manager,JGiola/swift-package-manager,mxcl/swift-package-manager,mxcl/swift-package-manager,vmanot/swift-package-manager,abertelrud/swift-package-manager,vmanot/swift-package-manager,sschiau/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,apple/swift-package-manager,haskellswift/swift-package-manager,JGiola/swift-package-manager,haskellswift/swift-package-manager,haskellswift/swift-package-manager,JGiola/swift-package-manager