Show Package Content option not available in High Sierra on a pkg file
The Show Package Contents is used to show the contents of an application. If you go to an application on you mac and right click on it, you get a menu and one option is to "Show Package Contents". This applies to apps.

There is a way to show the contents of a pkg file but you must do it using the terminal.  Just type the following command: pkgutil --expand /path/to/package.pkg /output/destination/. A description of this is given here.