Also factor out the details since we would otherwise have four instances of
them now.

Maybe it always should be?  But configuring only PYTHONPATH is strictly easier
than configuring PATH and PYTHONPATH so I guess we'll let people do that.

By the time this code runs we know PYTHONPATH must be configured correctly
since we import stuff from allmydata elsewhere.

This helps with debugging failures.
Also the directory deletion implementation is broken on Windows anyway and
sometimes fails with errors like:

```
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'C:\\Users\\RUNNER~1\\AppData\\Local\\Temp\\tmpc8za73y6\\private\\privatestorageio-zkapauthz-v1.sqlite3'
```

failing because somebody leaks a Client into the reactor