Minor refactoring

[maresb]

Here are a few really minor refactoring things I did in preparation for vendoring Poetry in #240, which should be very straightforward to review.

1. Fix typo in docs: pip was listed twice as a dependency in CONTRIBUTING.md.
2. The variable dep was being used as both Dependency and LockedDependency types, so to make type checkers happier, I renamed the LockedDependency instances of dep to locked_dep.
3. When initializing the HashModel object, it is possible (from the typing perspective if not in practice) that link.hash_name and link.hash are None. Thus HashModel(**{link.hash_name: link.hash}) would expand to HashModel(None=None), which would result in TypeError: keywords must be strings. I rewrote this bit of code to be a bit lengthier, but more explicit and robust.

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