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/Dockerfile

@@ -0,0 +1,15 @@
+FROM node:12-alpine
+
+WORKDIR /app
+
+COPY package.json package.json
+
+RUN npm install
+
+COPY . .
+
+ENV PORT=8080
+
+EXPOSE 8080
+
+CMD ["npm", "start"]