env: default to enabling pip/wheels/setuptools

For project virtual environments, default to enabling pip, setuptools
and wheel packages to retain existing stable behaviour to prevent
unexpected breakages caused by development environments making
assumptions of base package availability in virtual environments.

Poetry itself does not require the use of these packages and will
execute correctly within environments that do not have these packages.

This change retains the ability to manage these packages as direct
project dependency as introduced in #2826. All poetry internal
execution of pip is retaining the use of the wheel embedded within
the virtualenv package used by poetry.

In cases where a one of these reserved packages are being managed as a
project dependency, the will be treated as any other project
dependency. Executing `poetry install --remove-untracked` will not
remove any of these reserved packages. However, `poetry add pip` and
`poetry remove pip` will trigger the update and removal of `pip`
respectively.

Relates-to: #2826
Relates-to: #3916