loader.metadata.github: Use directly core.utils.github.GitHubSession

This allows to drop one unneeded dependency on lister (which creates build issues currently [1]).

loader-{git,save-code-now,add-forge-now,...} images -depend-> swh-loader-metadata -depend-> swh-lister (with a new deps rpy2 which breaks build).

Dropping this deps allows to simplify this graph to:

loader-{git,save-code-now,add-forge-now,...} images -depend-> swh-loader-metadata

Hence fixing the builds for all those images currently failing but the lister (which i fixed locally already)

Refs. swh/infra/sysadm-environment#5017 (closed)

[1] https://jenkins.softwareheritage.org/job/swh-apps/job/build-docker-image/518/console (as one example)