[WIP] Fix for trait-guarded deps being included in resolution

[bripeticca]

Trait-guarded dependencies were still being considered during dependency resolution, when they should be excluded if they aren't being used in any other scenario.

Modifications:

Since we have the --experimental-prune-unused-dependencies feature behind an experimental flag, we'll now consider an alternate path that will prune trait-guarded package dependencies from the dependency graph if and only if said dependency is not used in any other unguarded context.

Result:

Trait-guarded dependencies are excluded from dependency resolution.

[bripeticca]

@swift-ci please test