remove use of hardcoded source from Common class in DAMappingElementFinder

also renamed PsiClassIndex implementations and abstract class for clarity about their content and purpose
move PsiClass generation out of PsiElementFinder
refactor source generation to simplify coding and open door for extension

FileGeneratorContext renamed to GenerationContext and now only holds data, no computing at all
moved computing of GenerationContext to new service : GenerationContextComputer
introduced GeneratedFileDescriptor, associated to a String key, to represent each generated file
each GeneratedFileDescriptor has the access to the GenerationContext it belongs to and knows its key
this drastically simplifies coding the generation of one file a time (for example in ParseAngGenerateManager
this allows the implementation of PsiShortNameCache without any hardcoded stuff