Fix AutoPlugins declared at top-level

When having `AutoPlugin` declared as top-level classes, an exception is thrown during build load. This pull request is an attempt to fix it. 

The top-level `AutoPlugin` classes are not imported any more, and `autoImport` statements of top level plugins are not prefixed with `_root_`.

```
import _root_.sbt.plugins.IvyPlugin, _root_.sbt.plugins.JvmPlugin, _root_.sbt.plugins.CorePlugin, _root_.sbt.plugins.JUnitXmlReportPlugin, MyPlugin
                                                                                                                                                   ^
sbt.compiler.EvalException: Error parsing imports for expression.
	at sbt.compiler.Eval.checkError(Eval.scala:343)
	at sbt.compiler.Eval.sbt$compiler$Eval$$parseImport(Eval.scala:295)
	at sbt.compiler.Eval$$anonfun$parseImports$1.apply(Eval.scala:289)
	at sbt.compiler.Eval$$anonfun$parseImports$1.apply(Eval.scala:289)
	at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
	at scala.collection.TraversableLike$$anonfun$flatMap$1.apply(TraversableLike.scala:251)
```

This is a resend of #1365.