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_unix.go

@@ -12,9 +12,9 @@ import (
 )
 
 func dialService(service *plancontext.Service) (net.Conn, error) {
-	if service.Unix == "" {
+	if service.Unix() == "" {
 		return nil, errors.New("unsupported socket type")
 	}
 
-	return net.DialTimeout("unix", service.Unix, time.Second)
+	return net.DialTimeout("unix", service.Unix(), time.Second)
 }