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.
Initialize it globally and turn it off where needed, instead of enabling
it on demand. Also fixes void-function: flycheck-mode errors when
:feature syntax-checker is disabled. This is experimental.
Indirectly fixes#710
