Add support for inter-plugin dependencies

With a growing number of Essentials plugins, the need for a dependency mechanism between plugins (plugin A requires to be installed only once plugin B is in place) is getting bigger. A simple example is the conversion from community to enterprise, which is now triggered magically, but which also produces the somewhat quirky Experience Project plugin (ESSENTIALS-1115).

Such a mechanism should support

• transitive dependencies (A depends on B, which depends on C)
• multiple dependencies (A depends on both B and C)
• rebuild constraints per dependency (in order to install A, B needs to be installed AND a rebuild must (or needs not) have happened)