fix project saving and reading of vsizip paths (fixes #6369)

I am submitting this as a pull request for review because it can potentially break layer reading/writing in projects. 

But it should not, as it simply removes the vsiprefix from a path (e.g. /vsizip/) before prepending it to final result, in QgsProject::readPath writePath

QgsZipItem::vsiPrefix was moved to Qgis.h so that any code that uses it does not depend on qgsdataitem.h