embedded stdlib

This PR embeds the stdlib into the dagger binary itself for convenience.
Long term we will want to source the stdlib directly from git.

- Updated go to 1.16 to use the new built-in embedding functionality
- The `stdlib` go package now contains an embed of the stdlib

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

stdlib/stdlib.go

@@ -0,0 +1,49 @@
+package stdlib
+
+import (
+	"embed"
+	"fmt"
+	"io/fs"
+	"path"
+	"path/filepath"
+
+	cueload "cuelang.org/go/cue/load"
+)
+
+// FS contains the filesystem of the stdlib.
+//go:embed **/*.cue
+var FS embed.FS
+
+const (
+	stdlibPackageName = "dagger.cloud"
+)
+
+func Overlay(prefixPath string) (map[string]cueload.Source, error) {
+	overlay := map[string]cueload.Source{}
+
+	err := 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(p)
+		if err != nil {
+			return fmt.Errorf("%s: %w", p, err)
+		}
+
+		overlayPath := path.Join(prefixPath, "cue.mod", "pkg", stdlibPackageName, p)
+
+		overlay[overlayPath] = cueload.FromBytes(contents)
+		return nil
+	})
+
+	return overlay, err
+}