distro: don't fall back to fedora-30

Make osbuild-composer use FromHost() directly. Everywhere else needs to
specify the distro explicitly.

Also don't panic when a distro doesn't exist. Instead, return nil. Make
sure all callers check for that.

cmd/osbuild-pipeline/main.go

@@ -32,6 +32,10 @@ func main() {
 	}
 
 	d := distro.New(distroArg)
+	if d == nil {
+		panic("unknown distro: " + distroArg)
+	}
+
 	pipeline, err := d.Pipeline(blueprint, format)
 	if err != nil {
 		panic(err.Error())