`--dev` option not exporting dev dependencies

The deprecated --dev option does not work as intended: when running with --dev, the dev dependencies are not being exported.

Running poetry export with or without the --dev option won't make a difference:

poetry export --dev --format=requirements.txt --output=requirements-dev.txt
poetry export --format=requirements.txt --output=requirements.txt
diff requirements.txt  # no output

Steps to reproduce:

1. Create a pyproject.toml file with dev dependencies
2. Run poetry export --dev --format --requirements.txt --output=requirements-dev.txt
3. Notice that the output file does not include any dev dependencies

Example pyproject.toml

[project]
name = "poetry-demo"
version = "0.1.0"
requires-python = ">=3.10"
dependencies = [
]


[build-system]
requires = ["poetry-core>=2.0.0,<3.0.0"]
build-backend = "poetry.core.masonry.api"

[tool.poetry.group.dev.dependencies]
pytest = "^8.3.4"

[tool.poetry.requires-plugins]
poetry-plugin-export = ">=1.9"

I know that the dev dependencies can be easily exported with the --with=dev option, however the --dev, although deprecated, should still work.

Poetry version: 2.0.1
poetry-plugin-export version: 1.9.0

Start by reproducing the issue with the provided pyproject.toml and the poetry export commands, then trace how the deprecated --dev option is handled. Add a regression test covering --dev and verify that the generated requirements-dev.txt includes the dev dependency while the regular export does not.