golang: Ensure package version is uppercase encoded in download URL

While the package version was properly uppercase encoded in the URL for fetching package version info, it was not in the download URL resulting in 404 and partial visit of the golang origin.

swh/loader/package/golang/tests/data/https_proxy.golang.org/github.com_adam-hanna_array!operations_@v_list

 v1.0.1
+v1.0.1-RC1