src/poky:bitbake: fetch2/gomod: Fix mirroring problem

Build the 'downloadfilename' parameter by replacing path separators in
the module path like the git fetcher builds the mirror tar ball name.
Copy the downloaded file in the fetcher's unpack method like the crate
fetcher instead of calling the base fetcher's unpack method.

(Bitbake rev: 7762cea087597019460d66b04268757bd46befdf)

Signed-off-by: Christian Lindeberg <christian.lindebergaxis.com>
Signed-off-by: Richard Purdie <richard.purdielinuxfoundation.org>

Author: chrislindeberg