This is a port of 53221078e0de to Windows to allow pip-installed extensions to
be loaded without specifying a path. It's a major headache to have an hg.exe on
PATH that needs to have the path to the extensions specified, because WSL
doesn't see the same path.
This is only for Windows for now, to match the currently shipping py2 behavior.
There is a better solution with using the site package, but this needs support