Calling this pivotal macro "def-package!" has frequently been a source
of confusion. It is a thin wrapper around use-package, and it should be
obvious that it is so. For this reason, and to match the naming
convention used with other convenience macros/wrappers, it is now
use-package!.
Also changes def-package-hook! -> use-package-hook!
The old macros are now marked obsolete and will be removed when straight
integration is merged.
