For searching within RFC modules, you can use the transaction BAPI (Business Application Programming Interface) for searching modules. The advantage of this approach is that they are completely documented and available on SAP website with sample usage examples. Also, SAP provides support for the same and in case you are stuck somewhere, SAP support will be a good hand for guidance.
Also, you can use transaction SE80, SE84 to query for modules, programs. Search using this supports the search for names, id, and descriptions.
You can also use RS_ABAP_SOURCE_SCAN for searching across ABAP programs. It works fine with sub-system search but for the entire system, the search might be slow.
You can also use token search available in SAP code inspector.