Function to retrieve package version

[Paul W. Rankin]

wrt. 
http://lists.gnu.org/archive/html/help-gnu-emacs/2019-05/msg00022.html

I would like an easy function that retrieves the version of the 
current package, e.g.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
;;; my-great-package.el --- A great package

;; Copyright (c) 1952-2019  Free Software Foundation, Inc.

;; Author: Mr. Big Pants <address@hidden>
;; Version: 2.7.0
;; Package-Requires: ((emacs "24.5"))

(defvar my-great-package-version
 (package-get-version))
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   my-great-package-version
   -> "2.7.0"

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