Refactor PMCDeposit activity

[Graham Nott]

Recently a small bug in PMC deposits was observed, where it didn't find the article XML file correctly, since the article had more than one .xml files in the zip package.

I am fixing that bug, and the code for this activity looks like it can be improved further.

Todo

• Fix the bug for finding the correct article XML file
• Refactor how the activity directories / folders are used
• Use file functions from the article_processing provider
• Refactor for functions with no self-use
• Remove all the if self.logger conditional statements
• Use the ftp.py provider for FTP functions
• Switch all S3 operations over to use storage context from the storage_provider
• More refactoring for better tests
• Increase code coverage
• Code linting

[Graham Nott]

Steps are completed in PR #930.