engine: Support plan outputs

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

tests/plan/outputs/.gitignore

@@ -0,0 +1 @@
+out

tests/plan/outputs/outputs.cue

@@ -0,0 +1,21 @@
+package main
+
+import "alpha.dagger.io/europa/dagger/engine"
+
+engine.#Plan & {
+	actions: {
+		scratch: engine.#Scratch
+
+		data: engine.#WriteFile & {
+			input:    scratch.output
+			path:     "/test"
+			mode:     0o600
+			contents: "foobar"
+		}
+	}
+
+	outputs: directories: test: {
+		contents: actions.data.output
+		dest:     "./out"
+	}
+}