Distr opts improvements base

[KOLANICH]

Problem

For #2034 and #2036 we need to load pyproject.toml. To do it we need to know the path of the dir where it is situated. We also want this path to be used to load setup.cfg properly. We also don't want to do these ops if their result is unneeded, since they are a bit costly.

Summary of changes

• Implemented ''get_setup_py_path'' that allows to get a path of ''setup.py'' file.
• Implemented ''get_setup_py_dir'' that allows to get a path of a dir where''setup.py'' file resides.
• Implemented ''read_toml'' that just reads a TOML file. Needs ''toml' lib.
• Implemented ''read_pyproject_toml'' that just reads ''pyproject.toml' file.
• Implemented ''FinalizeDistributionOptionsHookArgsCache'' that does these operations lazily and caches their results, to use them for hooks in future.

Pull Request Checklist

• Changes have tests
• News fragment added in changelog.d. See documentation for details