Currently the only supported directories for the Pipeline Shared Groovy Library plugin are "src" and "vars". It would be helpful if directories for storing configuration variables and unit tests were also supported. As stated in the plugin documentation "Other directories under the root are reserved for future enhancements.", so for reasons of forwards compatibility it makes sense for developers not to assume folders they create for these purposes won't break their libraries in future versions.
Perhaps the "src" directory could support a more standard layout such as "src/main" , "src/test", "src/resources"? Or perhaps new directories at the top level such as "conf" and "test"?
The "test" would be for unit testing the code in "src" and "vars". The "conf" directory would be used for configuration files such as YAML or JSON where credential IDs, job names etc. could be stored, decoupled from the code.