fix: scope hatch packages directive to wheel target only

[jonburdo]

The packages directive was under [tool.hatch.build] (general), which applies to both wheel and sdist targets. Hatchling's sdist builder flattens the src-layout, placing model_signing/ at the archive root, but the embedded pyproject.toml still references src/model_signing - a path that no longer exists in the sdist. This causes pip to install an empty wheel with zero Python modules.

Moving the directive to [tool.hatch.build.targets.wheel] lets the sdist target use hatchling's default behavior (preserve the full source tree including the src/ prefix), so the packages path matches the actual layout when installing from source distributions.

Resolves: #636

Summary

This allows installation from sdist. I've tested with a script:
test-sdist.sh

#!/bin/bash
# Test whether rh-model-signing installs correctly from sdist.
# Usage: ./test-sdist.sh <branch>
set -euo pipefail

branch=${1}

git checkout "$branch" --quiet
rm -rf dist/ .venv-sdist-test
hatch build -t sdist
python -m venv .venv-sdist-test
.venv-sdist-test/bin/pip install dist/*.tar.gz --no-cache-dir --quiet
.venv-sdist-test/bin/python -c "import model_signing; print('PASS')"
rm -rf dist/ .venv-sdist-test

This fails on the current main (7643e29):

$ ./test-sdist.sh main
──────────────────────────────────────────────────────────────────────────────────────────── sdist ─────────────────────────────────────────────────────────────────────────────────────────────
dist/model_signing-1.1.1.tar.gz

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import model_signing; print('PASS')
    ^^^^^^^^^^^^^^^^^^^^
ModuleNotFoundError: No module named 'model_signing'

But is fixed on this branch:

$ ./test-sdist.sh fix/sdist-build-config-sigstore
──────────────────────────────────────────────────────────────────────────────────────────── sdist ─────────────────────────────────────────────────────────────────────────────────────────────
dist/model_signing-1.1.1.tar.gz

PASS

Checklist

• All commits are signed-off, using DCO
• All new code has docstrings and type annotations
• All new code is covered by tests. Aim for at least 90% coverage. CI is configured to highlight lines not covered by tests.
• Public facing changes are paired with documentation changes
• Release note has been added to CHANGELOG.md if needed

[mihaimaruseac]

Thank you