unpack sources where name is not the pkg

for instance, I want to add libcdio-paranoia, the package is:
PKG_URL="https://github.com/rocky/libcdio-paranoia/archive/master.zip"

we also need to add the following parameter to unpack it correctly:
PKG_SOURCE_DIR="libcdio-paranoia/libcdio-paranoia-master"

this hack avoid to create a special unpack function in package.mk