Add FrozenRequirement._init_args_from_dist() helper method

[cjerdonek]

This refactors from FrozenRequirement.from_dist() a helper class method I'm calling FrozenRequirement._init_args_from_dist().

This will be useful for issue #5031 (and independently of that issue) because it will let us use guard clauses to reduce indentation, cut down on the number of if-elses, and possibly remove some of the boolean "flag" variables. (Issue #5031 will likely require adding another if clause, which is what sparked this PR.)

[cjerdonek]

@pradyunsg Can you review this one? It's very easy and is a dependency for #5031. Thanks!

[cjerdonek]

@uranusjr Would you mind reviewing this simple PR for another pair of eyes?

[uranusjr]

I wonder if those flags and if-elses can be implemented cleaner with subclasses, but this helper method would make it easier to override behaviours in subclasses for that approach as well.

[cjerdonek]

Thanks!