Stubbed out new plugins {} block in build, settings and init scripts.

This delegates to a _script_ owned PluginHandler instance, which at the moment does nothing.
Liberate some utility methods from being instance methods on an abstract base class.
Better variable name.
Some detangling in the script transform infrastructure.

This is both cleanup for cleanups sake to try and make things easier to understand, and preparation for upcoming changes.
Factor helper method out as a static util.
Factor another helper method out as a static util, and fix visibility of util methods.
Turn unnecessarily instance inner to static inner.
Some more decoupling in the script transform infrastructure.
Remove an unnecessary inheritance layer in the transform infrastructure.
Execute the plugins {} block in the same pass as the buildscript {} block.

Some var names are now not quite accurate. Will be cleaned up in pending commit.