TO SPLIT: Implement IDE modules
Introduce MultiModuleAnalysisTest

Checks that analysis produces expected diagnostics when configured by real IDEA Module
Remove additional logic for comparing files in JavaClassFinderImpl
ResolveSessionForBodies is not ModificationTracker
TEMP: some scope magic for profiling
Introduce packageExists utility in PackageIndexUtil
Use PackageIndexUtil#packageExists in IDELightClassGenerationSupport
Introduce packageExists utility in PluginDeclarationProviderFactory
FIXUP: remove unoptimal code in JetSourceFilterScope
++ Use PackageIndexUtil#packageExists in IDELightClassGenerationSupport
Improve SyntheticFilesFilteringScope performance
COMMENT: JetSourceFilterScope performance
DRAFT: ATTEMPT at packages indices
DIRTY: remove erroneus JetSubPackagesIndex
DRAFT SubpackagesIndexService.kt
++
Use SubpackagesIndexService
Use SubpackagesIndexService in plugin.xml
Use JetExactPackagesIndex in getAllPossiblePackageClasses
++ JetExactPackagesIndex