Simply pass through packages starting with "^" or containing "*". For this
cycle we can trust our distributors won't do anything unusual, but next cycle,
we need to perform explicit validation on these items.
If you are a downstream, please be careful!
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.
