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>

solver/socketprovider_windows.go

@@ -13,10 +13,10 @@ import (
 )
 
 func dialService(service *plancontext.Service) (net.Conn, error) {
-	if service.Npipe == "" {
+	if service.NPipe() == "" {
 		return nil, errors.New("unsupported socket type")
 	}
 
 	dur := time.Second
-	return winio.DialPipe(service.Npipe, &dur)
+	return winio.DialPipe(service.NPipe(), &dur)
 }