Clean up major mode check in package-menu-mode

Clean up major mode check in package-menu-mode

* lisp/emacs-lisp/package.el (package--ensure-package-menu-mode):
Extract function to warn about incorrect major mode...
(package-menu-toggle-hiding, package-menu-refresh)
(package-menu-execute): ...from here.
(package-menu--mark-upgrades-1): And here, but move call...
(package-menu-mark-upgrades): ...here instead.
(package-menu-hide-package, package-menu-mark-delete)
(package-menu-mark-install, package-menu-mark-unmark)
(package-menu-quick-help, package-menu-get-status)
(package-menu-filter-by-keyword, package-menu-filter-by-name)
(package-menu-clear-filter): Add call to new function.  (Bug#37891)