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/go/tests/go.cue

@@ -0,0 +1,33 @@
+package go
+
+import (
+	"alpha.dagger.io/dagger"
+	"alpha.dagger.io/alpine"
+	"alpha.dagger.io/dagger/op"
+)
+
+TestData: dagger.#Artifact @dagger(input)
+
+TestGoBuild: {
+	build: #Build & {
+		source: TestData
+		output: "/bin/testbin"
+	}
+
+	test: #up: [
+		op.#Load & {from: alpine.#Image},
+		op.#Exec & {
+			args: [
+				"sh",
+				"-ec",
+				"""
+					test "$(/bin/testbin)" = "hello world"
+					""",
+			]
+			mount: "/bin/testbin": {
+				from: build
+				path: "/bin/testbin"
+			}
+		},
+	]
+}

pkg/alpha.dagger.io/go/tests/testdata/go.mod

@@ -0,0 +1,3 @@
+module main
+
+go 1.14

pkg/alpha.dagger.io/go/tests/testdata/main.go

@@ -0,0 +1,7 @@
+package main
+
+import "fmt"
+
+func main() {
+	fmt.Println("hello world")
+}