package loader: crash on dangling symlinks

https://sentry.softwareheritage.org/organizations/swh/issues/104805/?referrer=phabricator_plugin

KeyError: "linkname 'domain-entity/SUMMARY.md' not found"
(7 additional frame(s) were not displayed)
...
  File "tarfile.py", line 2049, in extractall
    self.extract(tarinfo, path, set_attrs=not tarinfo.isdir(),
  File "tarfile.py", line 2090, in extract
    self._extract_member(tarinfo, os.path.join(path, tarinfo.name),
  File "tarfile.py", line 2171, in _extract_member
    self.makelink(tarinfo, targetpath)
  File "tarfile.py", line 2262, in makelink
    self._extract_member(self._find_link_target(tarinfo),
  File "tarfile.py", line 2440, in _find_link_target
    raise KeyError("linkname %r not found" % linkname)

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Migrated from T4638 (view on Phabricator)