Lockfile-Related Code Refactored Before Sourcefile PR

[srilman]

This PR is a piece of the refactoring done in PR #316. This PR

• Creates a new module and folder called 'conda_lock/lockfile` containing lockfile-related code
• Move the contents of conda_lock/src_parser/lockfile.py to conda_lock/lockfile/__init__.py (the read and write functions)
• Move the data structures related to lockfiles from conda_lock/src_parser/__init__.py to conda_lock/lockfile/models.py
• Move the _apply_categories to conda_lock/lockfile/__init__.py
• And any misc. changes

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[mariusvniekerk]

That's a lot for this one, makes the next one much easier to handle