Set %_pyproject_wheeldir and %_pyproject_builddir relative to the source tree, not $PWD
    
    This allows users to do:
    
        %build
        cd somewhere
        %pyproject_wheel
        cd -
        cd somewhere_else
        %pyproject_wheel
        cd -
    
        %install
        %pyproject_install
    
    Without a need to copy paste the wheels to a common location.
    
    This is in fact a breaking change, I'll make sure to adapt the affected packages in Fedora.