rhel85: distro and arch package sets

Add package sets to the top level distribution and the
architectures. Merge package sets when requested through
`imageType.PackageSets()`.
This allows us to have "cascading" package sets:
    distro > arch > image type

Previously this was only supported for the build packages. The idea is
generalised based on the idea of arbitrary package sets.

Certain package sets have special meaning (for the distro):
- packages: blueprint packages and special option packages are merged
  into this one.
- build: is not expected to be defined on the image type so it is merged
  from the distro and arch explicitly.

internal/rpmmd/repository.go

@@ -107,6 +107,14 @@ type PackageSet struct {
 	Exclude []string
 }
 
+// Append the Include and Exclude package list from another PackageSet and
+// return the result.
+func (ps PackageSet) Append(other PackageSet) PackageSet {
+	ps.Include = append(ps.Include, other.Include...)
+	ps.Exclude = append(ps.Exclude, other.Exclude...)
+	return ps
+}
+
 // TODO: the public API of this package should not be reused for serialization.
 type PackageSpec struct {
 	Name           string `json:"name"`