Add improved validation for package submissions/removals

[BrendanThompson]

As part of the Swift Mentorship Program @daveverwer and I have worked towards improving the validation stage(s) for package submission/removal.

• Provide better submitter feedback on exceptions during submission/removal failures
• Increased validation detection scenarios
• Ability to re-trigger CI on a package where validation has failed
• Alignment with the uplift security posture

These changes primarily enable validation of:

• Swift toolchain version validated against the base image (assumed good)
• Existence of a Package.swift
• Early dropout if incoming package exists in either packages.json or denylist.json

[daveverwer]

Thanks so much for all your hard work on this @BrendanThompson!

[finestructure]

Thanks for your work on this, @BrendanThompson !

As a lot of the changes are happening in the CI stage I'm wondering if there's a way to test this? Do you have a fork set up perhaps where the changes have been running or if not, would be feasible to set one up to demonstrate how it works?