cue native: environments can reference a module instead of embedding

one.

Fixes #631

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

tests/cli/packages/a/main.cue

@@ -0,0 +1,19 @@
+package a
+
+import "dagger.io/dagger/op"
+
+exp: {
+	string
+	#up: [
+		op.#FetchContainer & {ref: "busybox"},
+		op.#Exec & {
+			args: ["sh", "-c", """
+				printf a > /export
+				"""]
+		},
+		op.#Export & {
+			source: "/export"
+			format: "string"
+		},
+	]
+}

tests/cli/packages/b/main.cue

@@ -0,0 +1,19 @@
+package b
+
+import "dagger.io/dagger/op"
+
+exp: {
+	string
+	#up: [
+		op.#FetchContainer & {ref: "busybox"},
+		op.#Exec & {
+			args: ["sh", "-c", """
+				printf b > /export
+				"""]
+		},
+		op.#Export & {
+			source: "/export"
+			format: "string"
+		},
+	]
+}

tests/cli/packages/cue.mod/module.cue

@@ -0,0 +1 @@
+module: "dagger.io/test"