archive-loader: add support for .tar.Z and .tar.lz tarball types

Current run on archive loader (with gnu origins) makes apparent some missing tarball types support.

Examples follow (but possibly not limited to):

Nov 27 11:16:18 worker15 python3[13194]: [2019-11-27 11:16:18,693: ERROR/ForkPoolWorker-2] Fail to load https://ftp.gnu.org/gnu/gettext/
                                         Traceback (most recent call last):
                                           File "/usr/lib/python3/dist-packages/swh/loader/package/loader.py", line 299, in load
                                             dl_artifacts, dest=tmpdir)
                                           File "/usr/lib/python3/dist-packages/swh/loader/package/loader.py", line 212, in uncompress                                                                                                                                                      uncompress(a_path, dest=uncompressed_path)
                                           File "/usr/lib/python3/dist-packages/swh/core/tarball.py", line 164, in uncompress
                                             raise ValueError('File %s is not a supported archive.' % tarpath)
                                         ValueError: File /tmp/tmpx5y4rxia/gettext-0.19.4.tar.lz is not a supported archive.

Nov 27 11:18:13 worker12 python3[14766]: [2019-11-27 11:18:13,216: ERROR/ForkPoolWorker-2] Fail to load https://ftp.gnu.org/gnu/groff/
                                         Traceback (most recent call last):
                                           File "/usr/lib/python3/dist-packages/swh/loader/package/loader.py", line 299, in load
                                             dl_artifacts, dest=tmpdir)
                                           File "/usr/lib/python3/dist-packages/swh/loader/package/loader.py", line 212, in uncompress
                                             uncompress(a_path, dest=uncompressed_path)
                                           File "/usr/lib/python3/dist-packages/swh/core/tarball.py", line 164, in uncompress
                                             raise ValueError('File %s is not a supported archive.' % tarpath)
                                         ValueError: File /tmp/tmph7sqy64t/groff-1.02.tar.Z is not a supported archive.

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Migrated from T2110 (view on Phabricator)