Avoid using setuptools shim of distutils (#2305)
## Summary
It turns out that setuptools includes a shim to patch distutils. I'll
admit that I don't fully understand why or how it's different, but this
is the trick `pip` uses to ensure that it gets the "original" distutils.
We actually use distutils in two places: once for the system Python
scheme, and once for virtual environments. In virtualenv, they _do_ use
the patched distutils, so this could deviate in ways I don't understand.
Closes #2302.
