When we are creating the final list of packages to feed to "apt install,"
virtual packages which do not have a valid candidate are included in this
list, causing the installation to fail. When creating this list, use FindGrp
instead of FindPkg, and only add it to the list if there is a valid candidate.
Additionally, add these items to an unordered set, to eliminate deduplication
(LP: #2106773).
A reliable testcase for this is the libreoffice-help-is package, at least in
Plucky.
If checkpackage-backend does not exist in the target system, likely due to the
use of stacked squashfses, ensure it temporarily exists so it can be used in
the install process. This fixes installation of the Virtual Machine Manager
when selected on the Customize menu (LP: #2104243).
