While the xtask code base is better engineered than our slightly messy Nox
manifest, all functionality is available via Nox including some that is not
available via xtask. Finally, only the Nox sessions are used in the CI by now so
that xtask does not see regular automated usage.
* Fix Windows OSError
* Ignore .pyd files
* Put things in modules
* Rename functions to `run`
* Expand on cargo xtask
* Try to work around missing installs
* Run all things by default, but not llvm-cov
* Test msrv
* Fix more OSErrors
* Various refinements and docs
* Various refinements
* Various refinements