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NPM loader: "KeyError: 'version'" on some metadata files

https://sentry.softwareheritage.org/organizations/swh/issues/9039/?referrer=phabricator_plugin

KeyError: 'version'
  File "swh/loader/package/npm/tasks.py", line 14, in load_npm
    return NpmLoader.from_configfile(url=url).load()
  File "swh/loader/package/loader.py", line 584, in load
    )
  File "swh/loader/package/loader.py", line 585, in <listcomp>
    except Exception as e:
  File "swh/loader/package/npm/loader.py", line 140, in get_package_info
    project_metadata=self.info(), version=version
  File "swh/loader/package/npm/loader.py", line 63, in from_metadata
    extrinsic_version = package_metadata["version"]

Happened only on this project so far: https://www.npmjs.com/package/zachtestproject2

Migrated from T3977 (view on Phabricator)