universe vendoring

Rather than injecting universe at runtime, this change will vendor
alpha.dagger.io in `cue.mod` directly.

Fixes #700

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

tests/core/inputs-outputs/test-core.cue

@@ -0,0 +1,15 @@
+package testcore
+
+import (
+	"alpha.dagger.io/dagger"
+)
+
+name: dagger.#Input & {
+	string | *"world"
+}
+
+message: dagger.#Output & "Hello, \(name)!"
+
+dir: dagger.#Input & dagger.#Artifact
+
+samedir: dagger.#Output & dir