Query interpreter to determine correct `virtualenv` paths (#2188)
## Summary
This PR migrates our virtualenv creation from a setup that assumes prior
knowledge of the correct paths, to a technique borrowed from
`virtualenv` whereby we use `sysconfig` and `distutils` to determine the
paths. The general trick is to grab the expected paths with `sysconfig`,
then make them all relative, then make them absolute for a given
directory.
Closes #2095.
Closes #2153.
