Doesn't allow us to specify a directory path as an argument to submit_py_files parameter in spark_processor.run method

As per https://github.com/aws/sagemaker-python-sdk/blob/master/src/sagemaker/spark/processing.py#L76 the comments local directory paths should be allowed.

However, when we try to pass a directory path we get the following error:

`submit_deps path file:///home/ec2-user/SageMaker/code/ is not a valid local file. Please specify a list of one or more S3 URIs, local file paths, and/or local directory paths`

Most likely this spawns from the check below:

https://github.com/aws/sagemaker-python-sdk/blob/12a69180e2e88739f4fd28ed36fe88da40139f54/src/sagemaker/spark/processing.py#L441

`exists` check instead of isfile here might help resolve the issue.