Add an inline procedure for checking for source packages

Add an inline procedure for checking for source packages

* package.el (eval-when-compile): Require 'inline during compilation.
(package-vc-p): Add inline function.
(package-desc-full-name): Use it.
(package-load-descriptor): Use it.
(package--get-activatable-pkg): Use it.
(package-install-from-archive): Use it.
(package-update): Use it.
(package-desc-status): Use it.
(package--remove-hidden): Use it.
(package-menu--print-info-simple): Use it.