Possibility to add source_directory to Sagemaker.processing.ScriptProcessor

**Describe the feature you'd like**
Currently there is no option to add source_dir to `Sagemaker.processing.ScriptProcessor`, which makes it difficult to have modular code.

**How would this feature be used? Please describe.**
This will help using ScriptProcessor to be used for running complex processing logic. Especially where the project has different modules and each module perform different processing step.

**Describe alternatives you've considered**
running it directly on docker ?

Start by locating the Sagemaker.processing.ScriptProcessor entry point and reviewing how it currently packages or exposes processing scripts. Determine the expected source_directory behavior for modular processing code, then add coverage for that behavior and verify that ScriptProcessor can run code organized across modules.

Written by the indexing model from the issue text.