[BUG] grayskull seems to unnecessarily query `setup.cfg` / `setup.py` for non-setuptools build backends

[mgorny]

Describe the bug
According to the output, grayskull seems to be trying to find setup.cfg and setup.py files even when pyproject.toml specifies a completely different build backend. That seems unnecessary at best, and with potential to catch stale metadata at worst.

To Reproduce
grayskull gpep517

Expected behavior
grayskull noticing that the package specifies build-backend = "flit_core.buildapi" and ignoring setuptools files.

Outputs

Checking for pyproject.toml
pyproject.toml found in /tmp/grayskull-gpep517-kg7ky6od/gpep517-19/pyproject.toml
Recovering information from setup.py
Executing injected distutils...
Recovering metadata from setup.cfg
No data was recovered from setup.py. Forcing to execute the setup.py as script
Recovering metadata from setup.cfg

Environment:
pip list:

Package                Version
---------------------- -------------------------------
beautifulsoup4         4.14.3
certifi                2026.1.4
charset-normalizer     3.4.4
colorama               0.4.6
conda-souschef         2.2.3
coverage               7.13.1
execnet                2.1.2
grayskull              2.9.3.dev2+gf8a48b931.d20260113
idna                   3.11
iniconfig              2.3.0
mock                   5.2.0
numpy                  2.4.1
packaging              25.0
pip                    25.3
pkginfo                1.12.1.2
pluggy                 1.6.0
progressbar2           4.5.0
Pygments               2.19.2
pytest                 9.0.2
pytest-console-scripts 1.4.1
pytest-cov             7.0.0
pytest-mock            3.15.1
pytest-xdist           3.8.0
python-utils           3.9.1
RapidFuzz              3.14.3
requests               2.32.5
ruamel.yaml            0.19.1
ruamel.yaml.jinja2     0.2.7
setuptools             80.9.0
setuptools-scm         9.2.2
soupsieve              2.8.1
stdlib-list            0.12.0
tomli_w                1.2.0
typing_extensions      4.15.0
urllib3                2.6.3

Additional context
n/a

[marcelotrevisani]

yes, that is to try to cover most of the case scenarios, there are even projects that have a pyproject.toml set but uses a setup.py to build the package.
The variety of ways people are building packages differs a lot.
I hope someday everybody will use only pyproject.toml and everything will be easier, however, we still need to try to support some legacy

[mgorny]

But shouldn't we follow the behavior of Python installers like pip? I mean, if pyproject.toml specifies that another build backend is used, setup.py will effectively be ignored by pip and we'd be grabbing dead code.