### Bug Description Craft-parts assumes `prime` and `stage` entries are strings. This is not true if advanced grammar is used. Craft-parts needs to be able to unmarshal parts with advanced grammar. The problematic code is [here](https://github.com/canonical/craft-parts/blob/836a4fb7fbba405fc06e4da4d51117cdb99982b9/craft_parts/constraints.py#L35)). ### To Reproduce See a snapcraft reproducer in https://bugs.launchpad.net/snapcraft/+bug/2083964 ### part yaml ```shell parts: my-part: plugin: nil prime: - on amd64: - usr/lib/*/libvpl.so.* ``` ### Relevant log output ```shell 2024-10-09 09:12:00.397 Starting snapcraft, version 8.4.2 2024-10-09 09:12:00.397 Log verbosity level set to BRIEF 2024-10-09 09:12:00.397 Preparing application... 2024-10-09 09:12:00.397 Configuring application... 2024-10-09 09:12:00.398 Setting up ConfigService 2024-10-09 09:12:00.507 Build plan: platform=None, build_for=None 2024-10-09 09:12:00.507 Running snapcraft expand-extensions on host 2024-10-09 09:12:00.521 snapcraft internal error: AttributeError("'dict' object has no attribute 'startswith'") 2024-10-09 09:12:00.522 Traceback (most recent call last): 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_application/application.py", line 568, in run 2024-10-09 09:12:00.522 return_code = self._run_inner() 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/snapcraft/application.py", line 215, in _run_inner 2024-10-09 09:12:00.522 return_code = super()._run_inner() 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_application/application.py", line 549, in _run_inner 2024-10-09 09:12:00.522 return_code = dispatcher.run() or os.EX_OK 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_cli/dispatcher.py", line 487, in run 2024-10-09 09:12:00.522 return self._loaded_command.run(self._parsed_command_args) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/snapcraft/commands/extensions.py", line 137, in run 2024-10-09 09:12:00.522 project_data = models.Project.unmarshal(yaml_data_for_arch) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_application/models/base.py", line 61, in unmarshal 2024-10-09 09:12:00.522 return cls.model_validate(data) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/pydantic/main.py", line 568, in model_validate 2024-10-09 09:12:00.522 return cls.__pydantic_validator__.validate_python( 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_application/models/project.py", line 250, in _validate_part 2024-10-09 09:12:00.522 craft_parts.validate_part(part) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_parts/parts.py", line 648, in validate_part 2024-10-09 09:12:00.522 _get_part_spec(data) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_parts/parts.py", line 679, in _get_part_spec 2024-10-09 09:12:00.522 return PartSpec(**part_spec) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/pydantic/main.py", line 193, in __init__ 2024-10-09 09:12:00.522 self.__pydantic_validator__.validate_python(data, self_instance=self) 2024-10-09 09:12:00.522 File "/snap/snapcraft/12783/lib/python3.10/site-packages/craft_parts/constraints.py", line 44, in _validate_relative_path_str 2024-10-09 09:12:00.522 if path.startswith("/"): 2024-10-09 09:12:00.522 AttributeError: 'dict' object has no attribute 'startswith' 2024-10-09 09:12:00.522 Full execution log: '/home/developer/.local/state/snapcraft/log/snapcraft-20241009-091200.396666.log' ```