setup.py: Register git_checkout loader for use through CLI

It enables to use the loader through the following command.

$ swh loader run git_checkout ref= checkums=

Jenkins job DLDG/gitlab-builds #75 succeeded .
See Console Output and Coverage Report for more details.

And more than that, it allows to register the task type to the scheduler [1]. Which i had to overcome as it's badly named regarding the visit type anyway.

[1] swh/infra/sysadm-environment#4906 (comment 141018)

I'll probably do the equivalent mr about checking the tasks are named correctly and fail otherwise. Leading me to have to change that loader and task name...

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merged