Simplify revdep code

Isolate common code into a function and also fix a sublte bug hiding in
that code.

We need to stab a copy of the package inside our PackageMap to avoid
having symlinks pointing at the same values when iterating over
requires. ( e.g. happened in this case:
https://github.com/rancher-sandbox/cOS-toolkit/pull/467#issuecomment-895060115
)

pkg/package/package.go

@@ -136,6 +136,18 @@ type Packages []Package
 
 type DefaultPackages []*DefaultPackage
 
+type PackageMap map[string]Package
+
+func (pm PackageMap) String() string {
+	rr := []string{}
+	for _, r := range pm {
+
+		rr = append(rr, r.HumanReadableString())
+
+	}
+	return fmt.Sprint(rr)
+}
+
 func (d DefaultPackages) Hash(salt string) string {
 
 	overallFp := ""