[Get] Simplify packge version re-derivation.
[Get] Simplify packge version re-derivation.

 - This is still gross, but at least fixes it so that the computation is only
   done once, and in advance of the Package initialization.

 - NIFC

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