just-mr 'archive' repository: Absent roots sync with serve endpoint

To take advantage of absent roots, we need to ensure that a given
serve endpoint can build against the tree of this generated root.

To this end, for an 'archive' repository we only set the root as
absent if the serve endpoint knows already the root, it can set it
up itself, or we can create the root locally without a network
fetch and then upload it to the serve endpoint via the remote CAS.
A network fetch never gets performed for an absent root.

If a serve endpoint is not provided, an absent root can still be
generated, but only if no network fetches are required. In this
case a warning is emitted.

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