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
Use PackageIndexUtil#packageExists utility in PluginDeclarationProviderFactory
FIXUP: remove unoptimal code in JetSourceFilterScope
Improve SyntheticFilesFilteringScope performance
COMMENT: JetSourceFilterScope performance
DRAFT: ATTEMPT at packages indices
DRAFT SubpackagesIndexService.kt
Use SubpackagesIndexService
Use SubpackagesIndexService in plugin.xml
Use JetExactPackagesIndex in getAllPossiblePackageClasses
Extract containsAny utility
Use containsAny in SubpackagesIndexService
Add hasSubPackages to SubpackagesIndexService
Implement PackageIndexUtil#packageExists() via JetExactPackageIndex and SubpackagesIndexService