Align plugin to gradle-plugin-plugin

[britter]

Instead of trying to make this plugin work like maven plugin tools, this plugin should better work like gradle-plugin-plugin, becauses that's what Gradle users are used too:

• Instead of detecting mojos anywhere in the dependency graph, let users explicitly declare them in the build script like it's done with the GradlePluginDevelopmentExtension.
• A lot of the complexity of this plugin arises from trying to mimic Maven's project model using the Gradle model in order to feed it to maven plugin tools. Maybe throw away the whole maven plugin tools dependency and implement it's functionality from stratch