Stub out pm package create

pkg/pkg.go

@@ -0,0 +1,8 @@
+package pkg
+
+import "fmt"
+
+// Create traverses the contents of dir and emits a valid pkg, signed by id
+func Create(dir, id string) error {
+	return fmt.Errorf("creating package from %q for %q: NYI", dir, id)
+}