cue modules: move stdlib to pkg/alpha.dagger.io

In preparation for Europa, we will vendor multiple CUE modules:

- `pkg/alpha.dagger.io`: legacy non-europa packages
- `pkg/dagger.io`: core Europa packages
- `pkg/universe.dagger.io`: Europa universe

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

pkg/alpha.dagger.io/docker/compose/tests/testdata/docker-compose.yaml

@@ -0,0 +1,7 @@
+version: "3"
+
+services:
+    api:
+        build: ""
+        ports:
+        - 8080