Load class kind in KotlinSyntheticClass annotation

Fix some of the usages of KotlinSyntheticClass header where really package
parts were meant (they were committed by mistake).

DescriptorResolverUtils.isCompiledKotlinPackageClass no longer checks for class
being a compiled package part, because PSI classes are not built for package
parts (according to EmptyPackageFragmentClsStubBuilderFactory)
Minor, delete unnecessary parameter from util method
Minor, delete/inline methods
Extract DescriptorUtils.unwrapFakeOverride()
Minor, assert value != null in EnumValue

Similar to ArrayValue, AnnotationValue, etc.
Minor, revert unneeded "!!"
Add test on KotlinSyntheticClass annotation for $$TImpl
Delete default "import jet.*" from JS analyzer