Regression in plug-in loading

[dobey]

The change in revision 414 of lp:tarmac broke plug-in loading, from the internal plug-ins directory:

Traceback (most recent call last):
  File "bin/tarmac", line 6, in <module>
    main()
  File "/home/tarmac-atc/tarmac/tarmac/bin/__init__.py", line 30, in main
    registry.run(args)
  File "/home/tarmac-atc/tarmac/tarmac/bin/registry.py", line 60, in run
    self._run(args)
  File "/home/tarmac-atc/tarmac/tarmac/bin/registry.py", line 48, in _run
    run_bzr(args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 1131, in run_bzr
    ret = run(*run_argv)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 673, in run_argv_aliases
    return self.run(**all_cmd_args)
  File "/usr/lib/python2.7/dist-packages/bzrlib/commands.py", line 695, in run
    return self._operation.run_simple(*args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 136, in run_simple
    self.cleanups, self.func, *args, **kwargs)
  File "/usr/lib/python2.7/dist-packages/bzrlib/cleanup.py", line 166, in _do_with_cleanups
    result = func(*args, **kwargs)
  File "/home/tarmac-atc/tarmac/tarmac/bin/commands.py", line 365, in run
    load_plugins()
  File "/home/tarmac-atc/tarmac/tarmac/plugin.py", line 85, in load_plugins
    execfile(plugin_info[1], _module.__dict__)
IOError: [Errno 21] Is a directory: '/home/tarmac-atc/tarmac/tarmac/plugins/tests'