just-mr 'distdir' 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.

For a 'distdir' repository we can know the resulting tree
identifier directly without actually needing to fetch anything.
Therefore, we only set the root as absent if the serve endpoint
knows already this tree, if it can set it up itself, or if
we can provide this tree to the serve endpoint from one of our
CAS locations (local or remote), based on our tree invariant
guarantee. A network fetch of the archives 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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