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.go

@@ -36,7 +36,7 @@ func (sp *SocketProvider) ForwardAgent(stream sshforward.SSH_ForwardAgentServer)
 		id = v[0]
 	}
 
-	service := sp.pctx.Services.Get(plancontext.ContextKey(id))
+	service := sp.pctx.Services.Get(id)
 	if service == nil {
 		return fmt.Errorf("invalid socket id %q", id)
 	}