[WIP] Fix preemptive traits validation failure prior to package update

[bripeticca]

• Validating traits each time we compute transitively enabled traits/do a check as to whether this trait guards a dependency was impeding on swiftpm's ability to run the resolver and subsequently check for possible updates for dependency packages.
• Omit validation checks until after we have resolved/after we determine that resolution has determined that we have packages in need of updating.

TODO:

• assure that omitting validation in-between these stages doesn't cause us to do unnecessary checkouts; perhaps mark "problematic" packages?
• create tests that simulate the update process where trait configurations within package manifests have changed
• ensure the computations for requiredDependencies hasn't been changed re: now including dependencies we shouldn't (trait-guarded)