check_library_manager_compliance: Check for existing library in the index with same name

[per1234]

Each library added to the Library Manager index must have a unique library.properties name value.

The check would need to parse http://downloads.arduino.cc/libraries/library_index.json.

This must be a warning only, since check_library_manager_compliance may be used on libraries already in the Library Manager index to ensure their continued compliance.