Duplicate sub-plugins make general commands fail

[cemeceme]

If you have a plugin/sub-repo that internally uses another plugin (e.g. the cc-rules) and also have the parent project use both of these plugins, then commands such as plz build or plz test will fail with the error:
CRITICAL: Found multiple definitions for subrepo 'cc' ( [Details on the two instances of the sub-plugins] )

It is however, still possible to build a rule specifically (e.g.plz build //:ruleName) even though this rule would also have the same dependencies.