Backend import of local modules fails.

[nzlosh]

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I am...

• [x] Reporting a bug

I am running...

• Errbot version: 6.1.1
• OS version: alpine linux 3.8.4
• Python version: 3.6.8
• Using a virtual environment: yes

Issue description

Attempting to use mattermost backend fails to import its local modules with the following error.

(errbot) /opt/errbot # errbot -c /opt/errbot/mattermost_config.py 
08:31:24 INFO     errbot.bootstrap          Found Storage plugin: Shelf.
08:31:24 INFO     errbot.bootstrap          Found Backend plugin: Mattermost
08:31:24 DEBUG    errbot.storage            Opening storage 'repomgr'
08:31:24 DEBUG    errbot.storage.shelf      Open shelf storage /opt/errbot/data/repomgr.db
['/opt/errbot', '/opt/errbot', '/opt/errbot/bin', '/opt/errbot/lib/python36.zip', '/opt/errbot/lib/python3.6', '/opt/errbot/lib/python3.6/lib-dynload', '/usr/lib/python3.6', '/opt/errbot/lib/python3.6/site-packages', PosixPath('/opt/errbot/lib/python3.6/site-packages/errbot/storage'), PosixPath('/opt/errbot/backends/err-backend-mattermost')]
08:31:24 ERROR    errbot.bootstrap          Unable to load or configure the backend.
Traceback (most recent call last):
  File "/opt/errbot/lib/python3.6/site-packages/errbot/bootstrap.py", line 142, in setup_bot
    bot = backendpm.load_plugin()
  File "/opt/errbot/lib/python3.6/site-packages/errbot/backend_plugin_manager.py", line 49, in load_plugin
    plugin_classes = self.plugin_info.load_plugin_classes(self._base_module, self._base_class)
  File "/opt/errbot/lib/python3.6/site-packages/errbot/plugin_info.py", line 89, in load_plugin_classes
    spec.loader.exec_module(modu1e)
  File "<frozen importlib._bootstrap_external>", line 678, in exec_module
  File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
  File "/opt/errbot/backends/err-backend-mattermost/mattermost.py", line 21, in <module>
    from src.mattermostPerson import MattermostPerson
ModuleNotFoundError: No module named 'src'

Steps to reproduce

Perform standard errbot v6.1 installation. Install latest mattermost backend. Configure bot to start using mattermost.

Additional info

If you have any more information, please specify it here.