Handle Poetry Optional Dependency Quirks

[srilman]

Fixes #385. This is a compilation of some unusual behavior I noticed when conda-lock parses some dependencies in pyproject.toml files with conflicting attributes:

• When a dependency is an extra, but not specified as optional
• When a dependency is an extra, but not in [tool.poetry.dependencies]
• When a dependency is specified as optional, but not in [tool.poetry.dependencies]
• When a dependency is specified as optional and in [tool.poetry.dependencies], but is not part of any extra

There are all things that Poetry seems to handle quietly in its own way (see the related issue). To minimize the number of parsing changes, I did not change how conda-lock treats the resulting categories of dependencies.

• If a dependency is part of an extra, but not specified as optional or not under [tool.poetry.dependencies], conda-lock will print a warning but still treat it as part of the extra as it always has.
• If a dependency is marked as optional or not optional but isn't associated with an extra, conda-lock will print a warning but just presume that it is optional if not in the main category, required if in main.

Essentially, this PR ensures that we maintain the invariant that an optional source dependency is a dependency not part of the main category. This is important for future PRs implementing multiple category support.

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[maresb]

I'm really looking forward to this PR series, but unfortunately I have no time this week.

[srilman]

No worries @maresb. Always appreciate your reviews!