lib/db: get_package(): add flag resolve_virtual

Return the first provider of a virtual package if no real package with that
name exists. Disabled by default.

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: gracefully handle missing binary packages

These are usually udebs or binary packages for other architectures
that are listed in the Binary field of the source package.
Report them as unknown, but don't let them influence the overall
test result for that binary package.

Signed-off-by: Andreas Beckmann <anbe@debian.org>
p-r: properly report version of binary packages

which may be different from the version of their source package

Signed-off-by: Andreas Beckmann <anbe@debian.org>