fix(SerializedFile): load dependencies as dependencies

[nichinichisou0609]

SerializedFile.load_dependencies calls environment.load_file(path, True), which passes True as the parent argument of load_file, so is_dependency stays False. get_resource_data calls load_dependencies when the resource file of a texture, mesh or audio clip is not loaded yet, which then loads every external of the file as if the user had loaded it: their objects show up in Environment.objects next to the objects of the loaded file, and a loop over env.objects that runs again after reading such data also goes through the objects of the externals.

is_dependency=True is now passed by keyword, so the externals still resolve references and resources but are no longer listed as objects.

tests/test_dependencies.py (new) writes two copies of a sample serialized file to a temporary directory, the first listing the second as an external, loads the first and checks that load_dependencies() leaves Environment.objects as it was and marks the loaded external as a dependency.

Validation on Windows x86_64, CPython 3.11.13, pytest 9.1.1. The compiled path uses the UnityPyBoost extension of the 1.25.4 wheel; UnityPyBoost/ has no changes between 1.25.4 and 417d595. This branch and #383 merge without conflicts.

Check	Result
New test against unmodified 417d595	1 failed
Complete suite, with / without the extension	49 passed / 49 passed
ruff check and ruff format --check	Passed

python -m pytest -q tests/test_dependencies.py
python -m pytest -q tests