Refactoring on Source Parsing Related Functions

[srilman]

Description

This PR consists of some of the refactoring done in the first 2 commits of https://github.com/conda/conda-lock/pull/316. However, this PR applies some of the changes that were recommended in the comments of that PR. In total, this PR does the following:

• Move some of the Pydantic models from src_parser/init.py to the new src_parser/models.py
• Move the make_lock_spec and parse_source_files functions from conda_lock.py to src_parser/init.py
• Move the parse_pyproject_toml function to the end of pyproject_toml.py

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[maresb]

Sorry @srilman for the arduous process. At least I think we're substantially increasing the code quality with this PR.

I added some commits which I think make this ready. Could you please review them?

I believe I managed to resolve the circular import by moving aggregate_lock_specs to its own module. Now the imports can be loaded at top-level without errors. I also hope I resolved your concern regarding conda_lock.models by marking the helper functions as private (with a _ prefix).