We have a scons target called "document_sources_idl" that usually contains all .idl files related to document sources and relevant .cpp files with validators. Document source list search index is an exception since the validators are included with the full document source and are in the target "pipeline".  

We should move the validator into a separate file so that document_source_list_search.idl can be moved into the "document_sources_idl" target. 

See the relevant comment in Sconscript.