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/git/image.cue

@@ -0,0 +1,9 @@
+package git
+
+import (
+	"alpha.dagger.io/alpine"
+)
+
+#Image: alpine.#Image & {
+	package: git: true
+}