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/os/tests/os/os.cue

@@ -0,0 +1,25 @@
+package os
+
+import (
+	"alpha.dagger.io/alpine"
+)
+
+// Write a file to an empty dir
+EmptyDir: {
+	f: #File & {
+		path: "/foo.txt"
+		write: data: "hello world!"
+	}
+	f: contents: "hello world!"
+}
+
+// Read from a pre-existing file
+Read: {
+	f: #File & {
+		from: alpine.#Image & {
+			version: "3.15.0"
+		}
+		path: "/etc/alpine-release"
+	}
+	f: contents: "3.15.0\n"
+}