Expose base paths of `MultiplexedPath`

[FFY00]

I have run into the issue where I need to get the base paths of MultiplexedPath. Right now, my only option would be to check if the module I am passing to files is a namespace package, and if so, get the paths off it. It would be nice to expose the _path attribute of MultiplexedPath.

[henryiii]

Same need in scikit-build-core: scikit-build/scikit-build-core#896