plugins module should not depend on indexer module

[ahornace]

I think that we should introduce plugins-api module with the needed interfaces. I don't think that plugins module should be able to access indexer inner implementations.

[tulinkry]

Currently, these are available

50              "org.opengrok.indexer.configuration.Group",
51              "org.opengrok.indexer.configuration.Project",
52              "org.opengrok.indexer.configuration.RuntimeEnvironment",
53              "org.opengrok.indexer.authorization.IAuthorizationPlugin",
54              "org.opengrok.indexer.authorization.plugins.*",
55              "org.opengrok.indexer.authorization.AuthorizationException",
56              "org.opengrok.indexer.util.*",
57              "org.opengrok.indexer.logger.*",
58              "org.opengrok.indexer.Metrics"