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/tests/pull/pull.cue

@@ -0,0 +1,25 @@
+package docker
+
+import (
+	"alpha.dagger.io/dagger/op"
+	"alpha.dagger.io/alpine"
+)
+
+ref: string @dagger(input)
+
+TestPull: {
+	pull: #Pull & {from: ref}
+
+	check: #up: [
+		op.#Load & {from: alpine.#Image},
+		op.#Exec & {
+			always: true
+			args: [
+				"sh", "-c", """
+					 grep -q "test" /src/test.txt
+					""",
+			]
+			mount: "/src": from: pull
+		},
+	]
+}