Archive the pom file additionally to the source folder

Some archived maven origins do not contain the pom.xml file.

In these cases, the pom.xml should be archived as extrinsic metadata.

example : https://webapp.staging.swh.network/browse/origin/directory/?origin_url=https://repo1.maven.org/maven2/io/github/aliyun-beta/aliyun-slb

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Migrated from T4326 (view on Phabricator)