Add support for mojos from other projects

[britter]

It should be possible to have mojos implemented in a different subproject of a multi project build. When applied to a project the plugin should extract mojo descriptors form those projects, either via project dependencies or via a dedicated API:

mavenPlugin {
  mojos {
    from project("mojo-project") // extracts mojos from the main source set of that project
    from project("mojo-in-sourceset") {
      sourceSet = sourceSets.mojo // use the mojo source set from that project
    }
  }
}