Rework pyproject.toml parsing to never raise under setuptools

[twm]

This turns out to be a logic bug in how Incremental detects whether it has been enabled under setuptools. To quote the newsfragment:

Incremental could mis-identify that a project had opted in to version management.

If a pyproject.toml in the current directory contained a [project] table with a name key, but did not contain the opt-in [tool.incremental] table, Incremental would still treat the file as if the opt-in were present and attempt to validate the configuration. This could happen in contexts outside of packaging, such as when creating a virtualenv. When operating as a setuptools plugin Incremental now always ignores invalid configuration, such as configuration that doesn't match the content of the working directory.

The blast radius is nasty because many packaging tools use the distutils/setuptools APIs that invoke setuptools hooks.

This PR changes the rules for setuptools. Incremental now only activates itself if all of the following are true:

1. pyproject.toml contains a [tool.incremental] table
2. The project name can be extracted from pyproject.toml
3. A directory matching the project name is found in the current working directory
4. The directory matching the project name contains a _version.py file

In the Hatch case we continue to treat any configuration problems as errors because the Hatch plugin is only loaded when explicitly configured.

Additionally, this improves the integration test suite to isolate the build of the example projects and patches a coverage hole around _inf that brings us to 100% branch coverage.

Fixes #106.

[adiroiban]

Hi.

I did a very quick review... mostly just a placehoder review... in the case nobody else will have time for a full review.

Feel free to merge if another review is not provided in time.

Thanks

[twm]

Thank you for the review @adiroiban! I will proceed to merge and release as I am concerned that the issue this fixes will cause more problems in the ecosystem as the work week gets going.