Modify binary package name check to accept -dbgsym

 In particular, we should tolerate .deb packages that are in the
 Files, but are not present in the Binary package listing, since
 adding them into debian/control is a pretty bad idea.

 debhelper (currently, this will be default later), when givin
 `DH_BUILD_DDEBS=1` will build DH_BUILD_DDEBS that are in the Files
 list, named foo-dbgsym.

 The check now validates that the -dbgsym package corresponds to
 a package that *is* listed in the Binary package listing, and in
 the section "debug".