Running galaxy-importer in CI fails with doc fragments shared across collections

A collection can depend on the documentation fragments from another collection. However, galaxy-import does not pull these dependencies before it call ansible-doc.

It prepare a temp directory to isolate the collection which :

https://github.com/ansible/galaxy-importer/blob/master/galaxy_importer/loaders.py#L268

Examples:

https://bfcb0036379da16f975f-4282179313616d046bba66d871c7a9ef.ssl.cf5.rackcdn.com/579/16b009a77e4f66d679b0308972f29c92577bf4d3/check/ansible-galaxy-importer/bf50899/job-output.html#l779

See: https://github.com/ansible/ansible-zuul-jobs/issues/934