Kill implicit configuration: new configuration scheme

Collaborative document : https://hackmd.io/8hxTL4XMQoO2RVKtqFqM2g

Better config system that does not rely on implicit configurations.

Related:

swh-docs#1758 swh-docs#1388 #826 (closed) #1532 (closed) swh-deposit!90 (closed) !159 (closed) swh-indexer!456 (closed) !204 (closed)

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Migrated from T1410 (view on Phabricator)