Deprecate call to 'setup.py install' when 'wheel' is absent for source distributions without pyproject.toml

[sbidoul]

This is one of a series of deprecations meant to collect feedback on the ultimate goal of always installing source distributions by first building a wheel and then installing from it.

This specific issue is about the case when pip has to call setup.py bdist_wheel because the wheel package is not installed. In that case, pip currently displays deprecation warning and continues installation by attempting a setup.py install.

In a future version, pip will ~either (to be decided)~:

• ~fail right away~
• ~attempt setup.py bdist_wheel and let that fail (the error message might be unclear in that case)~
• attempt an isolated build in PEP 517 mode with the default build system

The recommended way to silence this warning is to enable the --use-pep517 flag, which will in turn install setuptools and wheel in an isolated environment where it will attempt to build a wheel and install it.

Towards #8102