move alpha.dagger.io/europa to dagger.io

Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>

pkg/dagger.io/dagger/plan.cue

@@ -0,0 +1,11 @@
+package dagger
+
+import (
+	"alpha.dagger.io/europa/dagger/engine"
+)
+
+// A deployment plan executed by `dagger up`
+#Plan: engine.#Plan
+
+// A special kind of program which `dagger` can execute.
+#DAG: engine.#DAG