Oh. Maybe I misunderstood the question. My understanding was that he was developing a terragrunt module and a terraform module that it depends on which live in different directories. So my scenario is that I have ~/work where infrastructure-modules and module-custom lives. So if I pass --terragrunt-source ~/work/infrastructure-modules//terragrunt-module but terragrunt-module needs to access my local development version of module-custom, I have to specify the absolute path of module-custom inside terragrunt-module. Does that make sense?
My latest experience came when trying to add aurora to my infrastructure-modules at the same time as adding monitoring_internal to the module-data-storage/modules/aurora.