Refactor common loader behavior within swh.model.from_disk.iter_directory function

This refactors the common duplicated code in svn [1] and package loader [2] within the model. That will decrease maintenance and also fix 1 todo \m/.

• [1] swh-loader-svn!37 (closed)

• [2] swh-loader-core!97 (closed)

Depends on !117 (closed)

Test Plan

tox

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Migrated from D3391 (view on Phabricator)