Bug fixes and tweaks

This release:

• renames --source-dir to --module-dir, to clarify that it should be the directory holding the module's sources;
• shows the before and after coverage relative to the source files requested, if any;
• fixes authentication error handling;
• switches to also using pytest-cleanslate for disabling polluting tests;
• fixes GPTv1 and Claude prompts not using filenames relative to the Python path, causing some LLMs to generate import statements starting with "lib" or "src";
• fixes a bug not ignoring import __main__ while detecting unavailable imports;