FUSE: directories referencing artifacts missing from the archive are reported as empty

$ ls archive/swh:1:dir:9e7cba45f1ef65a45647bce181bdf78f65da90a4
$ 

whereas the directory swh:1:dir:9e7cba45f1ef65a45647bce181bdf78f65da90a4 is not empty, but //references// a revision (as a submodule named "sha1collisiondetection") which is itself missing from the archive: swh:1:rev:855827c583bc30645ba427885caa40c5b81764d2.

Here are the logs:

ERROR:swh.fuse:Cannot fetch metadata for object swh:1:rev:855827c583bc30645ba427885caa40c5b81764d2: 404 Client Error: Not Found for url: https://archive.softwareheritage.org/api/1/revision/855827c583bc30645ba427885caa40c5b81764d2/
ERROR:swh.fuse:Cannot lookup: 404 Client Error: Not Found for url: https://archive.softwareheritage.org/api/1/revision/855827c583bc30645ba427885caa40c5b81764d2/
Traceback (most recent call last):
  File "/home/haltode/work/swh/swh-environment/swh-fuse/swh/fuse/fuse.py", line 290, in lookup
    lookup_entry = await parent_entry.lookup(name)
  File "/home/haltode/work/swh/swh-environment/swh-fuse/swh/fuse/fs/entry.py", line 104, in lookup
    async for entry in self.get_entries():
  File "/home/haltode/work/swh/swh-environment/swh-fuse/swh/fuse/fs/entry.py", line 93, in get_entries
    entries = [x async for x in self.compute_entries()]
  File "/home/haltode/work/swh/swh-environment/swh-fuse/swh/fuse/fs/entry.py", line 93, in <listcomp>
    entries = [x async for x in self.compute_entries()]
  File "/home/haltode/work/swh/swh-environment/swh-fuse/swh/fuse/fs/artifact.py", line 115, in compute_entries
    await self.fuse.get_metadata(swhid)
  File "/home/haltode/work/swh/swh-environment/swh-fuse/swh/fuse/fuse.py", line 87, in get_metadata
    metadata = await loop.run_in_executor(None, self.web_api.get, swhid, typify)
  File "/usr/lib/python3.8/concurrent/futures/thread.py", line 57, in run
    result = self.fn(*self.args, **self.kwargs)
  File "/home/haltode/work/swh/swh-environment/.venv/lib/python3.8/site-packages/swh/web/client/client.py", line 226, in get
    return self._getters[swhid_.object_type](swhid_, typify)
  File "/home/haltode/work/swh/swh-environment/.venv/lib/python3.8/site-packages/swh/web/client/client.py", line 306, in revision
    json = self._call(f"revision/{_get_swhid(swhid).object_id}/", **req_args).json()
  File "/home/haltode/work/swh/swh-environment/.venv/lib/python3.8/site-packages/swh/web/client/client.py", line 191, in _call
    r.raise_for_status()
  File "/home/haltode/work/swh/swh-environment/.venv/lib/python3.8/site-packages/requests/models.py", line 941, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://archive.softwareheritage.org/api/1/revision/855827c583bc30645ba427885caa40c5b81764d2/

Instead we should be able to list the directory entries and either report the symlink as broken or not show it at all.

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Migrated from T2851 (view on Phabricator)