Gradle plugin refactoring

1. Moved java-specific cross-compilation code to jvm task
2. Extracted most of the classes to "wrapper" module to make them accessible (as requested by [this commit comments thread](https://github.com/JetBrains/kotlin/commit/7e36547b3861bf9e59bd10fe4e87e19d2b95a9b8#commitcomment-8970287)). Also migrated tests.
Now core only contains classes that has direct dependencies on kotlin compiler API