Fix handle installed virtual packages

[kopp]

When a virtual package is requested by a rosdep.yaml file, that was not recognized as being installed because it is not found by dpkg-query. This patch searches for virtual packages using apt show and if it's a virtual package searches for the providing actual packages using apt-cache.

If python-apt (python api package for apt) is allowed as new dependency, this could be handled more easily -- see the comment in _is_installed_as_virtual_package.

[kopp]

This is a first step for #274: When a package providing a virtual package is present, the virtual package is considered installed. If you try to install a virtual package, apt should make sure that you get some package installed, that provides this virtual package.

[tfoote]

This looks valuable. Thanks for the contribution.

The additional dependency on python-apt might be reasonable. As long as it's available very generally on all platforms in a new enough version.

[wjwwood]

Thanks for the contribution @kopp. This looks good as is. My instinct is to merge it as is and track the option to use python-apt as an enhancement.