Improve Bitbucket plugin dependency behavior.

In certain situations, a 3rd party plugin can list an export for a package that is core to Bitbucket's internal system plugins. Disabling the 3rd party plugin will cause the dependency to be disabled along with Bitbucket's system plugins that also export that dependency. This will cause problems and prevent Bitbucket from starting.

We should:

1. Look at ways to prevent Bitbucket from failing (prevent plugins from starting if they attempt to export plugins that are exported by our system bundle)
2. Look at ways to make debugging similar issues in the future easier as the behavior on startup matches that of.