Revert "Refactor out read_pyproject_toml() and resolve_pyproject_toml()."
Revert "Refactor out read_pyproject_toml() and resolve_pyproject_toml()."

This reverts commit f66c1f7639bee37d1874c8c9e663da7cdbaf49cf.

rouge8/pip,pradyunsg/pip,pfmoore/pip,sbidoul/pip,pfmoore/pip,pradyunsg/pip,rouge8/pip,pypa/pip,xavfernandez/pip,xavfernandez/pip,pypa/pip,rouge8/pip,sbidoul/pip,xavfernandez/pip