We have way too many special cases in how we do installs. I just discovered a new one today - if the user doesn't have wheel installed, and is installing from source, and doesn't have a pyproject.toml, then we do setup.py install and bypass building a wheel. As a result (for example) the checks that installed script wrappers are on PATH don't get run.

We should reduce these special cases. It's likely we'll need some sort of deprecation or behaviour change warning, to ease migration, but ideally we should converge on a single installation path (which will be PEP 517, in due course).