sources can't extract tar.xz archive

[Olivier - interfaSys]

That's the trace from Snapcraft 2.8.4

```
"GET /files/transmission-2.92.tar.xz HTTP/1.1" 200 3378116
Traceback (most recent call last):
  File "/usr/bin/snapcraft", line 31, in <module>
    snapcraft.main.main()
  File "/usr/lib/python3/dist-packages/snapcraft/main.py", line 146, in main
    return run(args, project_options)
  File "/usr/lib/python3/dist-packages/snapcraft/main.py", line 193, in run
    lifecycle.snap(project_options, args['<directory>'], args['--output'])
  File "/usr/lib/python3/dist-packages/snapcraft/internal/lifecycle.py", line 240, in snap
    snap = execute('strip', project_options)
  File "/usr/lib/python3/dist-packages/snapcraft/internal/lifecycle.py", line 82, in execute
    _Executor(config, project_options).run(step, part_names)
  File "/usr/lib/python3/dist-packages/snapcraft/internal/lifecycle.py", line 111, in run
    self._run_step(step, part, part_names, dirty, recursed)
  File "/usr/lib/python3/dist-packages/snapcraft/internal/lifecycle.py", line 149, in _run_step
    getattr(part, step)()
  File "/usr/lib/python3/dist-packages/snapcraft/internal/pluginhandler.py", line 259, in pull
    self.code.pull()
  File "/usr/lib/python3/dist-packages/snapcraft/_baseplugin.py", line 120, in pull
    sources.get(self.sourcedir, self.build_basedir, self.options)
  File "/usr/lib/python3/dist-packages/snapcraft/internal/sources.py", line 280, in get
    handler.pull()
  File "/usr/lib/python3/dist-packages/snapcraft/internal/sources.py", line 188, in pull
    self.provision(self.source_dir)
  File "/usr/lib/python3/dist-packages/snapcraft/internal/sources.py", line 212, in provision
    self._extract(tarball, dst)
  File "/usr/lib/python3/dist-packages/snapcraft/internal/sources.py", line 247, in _extract
    tar.extractall(members=filter_members(tar), path=dst)
  File "/usr/lib/python3.5/tarfile.py", line 1995, in extractall
    numeric_owner=numeric_owner)
  File "/usr/lib/python3.5/tarfile.py", line 2037, in extract
    numeric_owner=numeric_owner)
  File "/usr/lib/python3.5/tarfile.py", line 2115, in _extract_member
    self.makelink(tarinfo, targetpath)
  File "/usr/lib/python3.5/tarfile.py", line 2202, in makelink
    self._extract_member(self._find_link_target(tarinfo),
  File "/usr/lib/python3.5/tarfile.py", line 2369, in _find_link_target
    raise KeyError("linkname %r not found" % linkname)
KeyError: "linkname 'transmission-2.92/qt/icons/Faenza/actions/scalable/small/list-remove.svg' not found"
```

But I can extract the archive just fine from the command line using the xf flags

The snapcraft config

```
  transmission:
    plugin: autotools
    source: https://download.transmissionbt.com/files/transmission-2.92.tar.xz
    source-type: tar

```

[Olivier - interfaSys (olivier-interfasys)]

Seems to be sym/hardlink related

https://github.com/python/cpython/blob/master/Lib/tarfile.py#L2357-L2374