Migrate dagger.#Secret and dagger.#Stream to new format

- Refactored to keep every transformation of built-in types (e.g. FS,
  Secret, etc) to/from CUE in the same place (plancontext)
- dagger.#Service and dagger.#Secret are now following the new FS-like format
  (e.g. `_service: id: string`)
- Backward compatibility
- dagger.#Stream is now an alias for dagger.#Service

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

plan/plan.go

@@ -67,10 +67,7 @@ func (p *Plan) registerLocalDirs() error {
 		if err != nil {
 			return err
 		}
-
-		p.context.LocalDirs.Register(&plancontext.LocalDir{
-			Path: dir,
-		})
+		p.context.LocalDirs.Add(dir)
 	}
 
 	return nil