MAINT improve isolation for micropip tests. I added a new `wheel_base` fixture which creates a new temporary directory and mocks `site.getsitepackages()` to return it. I also removed the `WHEEL_BASE` global variable from micropip and instead calculate it each time in `micropip.install`.
from ._micropip import _list as list
from ._micropip import freeze, install

__all__ = ["install", "list", "freeze"]

from ._micropip import _list as list
from ._micropip import freeze, install

__all__ = ["install", "list", "freeze"]
