distro: don't use a global variable to store the distro register

Introduce a DistroRegister object. For now this does not introduce
any functional changes, as the object is always instantited to be
the same. However, in follow-up patches it will get options.

Signed-off-by: Tom Gundersen <teg@jklm.no>

cmd/osbuild-composer/main.go

@@ -49,8 +49,9 @@ func main() {
 	jobListener := listeners[1]
 
 	rpm := rpmmd.NewRPMMD()
+	distros := distro.NewRegistry()
 
-	distribution, err := distro.FromHost()
+	distribution, err := distros.FromHost()
 	if err != nil {
 		log.Fatalf("Could not determine distro from host: " + err.Error())
 	}

cmd/osbuild-pipeline/main.go

@@ -34,7 +34,8 @@ func main() {
 		}
 	}
 
-	d := distro.New(distroArg)
+	distros := distro.NewRegistry()
+	d := distros.GetDistro(distroArg)
 	if d == nil {
 		panic("unknown distro: " + distroArg)
 	}