Support 'no-core' tarballs for profile downloads

[ergonlogic]

We currently have 2 ways to use profiles in Drush Make: either download just the profile, and use recursion to build out the rest, or using the profile as a core, which then bundles in the version of core that was current as of the latest release.

drupal.org publishes no-core tarballs too, but we don't currently appear to have a way to use them effectively. Since the core tarballs only get built when the distro/profile makes a release, there's no convenient way to use the tarballs when Drupal core gets a security release (for example), and thus requires a new release of the profile/distro.

It'd be nice to have a project option in Drush Make that supported the 'no-core' tarballs, as this would provide a clean work-around for this issue.

[ergonlogic]

This was surprisingly easy. See #2190