Remove `optional` From Dependency Info in Internal Code

[srilman]

This PR removes any references to an optional flag or option from the entire codebase. This builds on top of PR #388, so make sure to get that one merged first. At that point, there is an implicit invariant for all dependencies; the dependency is optional if category != "main". Therefore, storing the optional value just makes the code unnecessarily complex.

Note that this does not affect how source files are parsed or the produced lock file in any way. The step that writes the lockfile out will insert optional: true or optional: false flags depending on the category.

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[maresb]

Or does it make more sense to merge #390 all at once, once I've completed that?

[maresb]

~Ah, there are serious merge conflicts with #290 now in main. :disappointed:~

~It looks like you're both doing similar things, so I wonder if this also fixed the same bugs.~

Sorry, I seem to have been confused.

[srilman]

@maresb Resolving conflicts now ...

[srilman]

Merged with main, everything should be good!

[maresb]

Looking great, thanks!!!

As we wait for the Windows test to finish, can we rebase #390 on this?

[maresb]

The tests are passing. Let's pick up on https://github.com/conda/conda-lock/pull/389#discussion_r1199661413 with #390. I have to stop for tonight.