Extract pipeline out of __main__.py and into pipeline.py

This leaves __main__.py to handle command line arguments while pipeline.py
runs the pipeline - mostly. They are still somewhat intertwined, with
__main__.py doing essential things for pipeline.py, etc., and some
helper functions that could go in their own module.

All tests pass after this major refactor.

ocrmypdf/__init__.py

@@ -3,6 +3,8 @@ import os
 from collections.abc import Iterable
 import pkg_resources
 
+PROGRAM_NAME = 'ocrmypdf'
+
 VERSION = pkg_resources.get_distribution('ocrmypdf').version