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/pkg.go

@@ -0,0 +1,50 @@
+package pkg
+
+import (
+	"context"
+	"embed"
+	"fmt"
+	"io/fs"
+	"os"
+	"path"
+	"path/filepath"
+)
+
+var (
+	// FS contains the filesystem of the stdlib.
+	//go:embed alpha.dagger.io/**/*.cue alpha.dagger.io/**/*/*.cue alpha.dagger.io/europa/dagger/*.cue alpha.dagger.io/europa/dagger/engine/*.cue
+	FS embed.FS
+
+	AlphaModule   = "alpha.dagger.io"
+	EnginePackage = fmt.Sprintf("%s/europa/dagger/engine", AlphaModule)
+)
+
+func Vendor(ctx context.Context, dest string) error {
+	// Write the current version
+	return fs.WalkDir(FS, ".", func(p string, entry fs.DirEntry, err error) error {
+		if err != nil {
+			return err
+		}
+
+		if !entry.Type().IsRegular() {
+			return nil
+		}
+
+		if filepath.Ext(entry.Name()) != ".cue" {
+			return nil
+		}
+
+		contents, err := fs.ReadFile(FS, p)
+		if err != nil {
+			return fmt.Errorf("%s: %w", p, err)
+		}
+
+		overlayPath := path.Join(dest, p)
+
+		if err := os.MkdirAll(filepath.Dir(overlayPath), 0755); err != nil {
+			return err
+		}
+
+		return os.WriteFile(overlayPath, contents, 0600)
+	})
+}