check_snapshot: Drop accepting using dict for snapshot comparison

Note that it's ok within the loader-core (not yet for the other dvcs loaders).

This can land as is but then we'd need to release it one commit before that one (or in the mean time, we need to adapt the other dvcs loaders [1]).

Related to swh-storage#2494 (closed)

• [1] swh-loader-git!47 (closed) swh-loader-svn!44 (closed) swh-loader-mercurial!31 (closed)

Test Plan

tox

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Migrated from D3543 (view on Phabricator)