Refactor package.resovled check

[andrewdmontgomery]

Closes #642

Description

This unwinds #643 and approaches the challenge differently, using existing plugins from Danger and Buildkite.

Testing Steps

• Code review
• See the screenshots below for my testing and links to build log entries
• Optionally:
  1. Check out this branch at the commit before I reverted the TEST: Update... commit. [Check out: 9ea7e8b]
  2. Run danger locally against this PR at that commit
• Optionally:
  1. Create a branch from this branch
  2. Modify the Package.swift manually (without updating the Package.resolved (Ideally, this change would remove a plugin, in order to reproduce the events that led to this change, but I think any change is sufficient)
  3. Run danger

[wpmobilebot]

📲 You can test the changes from this Pull Request in Gravatar Prototype Build by scanning the QR code below to install the corresponding build.

	App Name	Gravatar Prototype Build
	Build Number	2070
	Version	1.0
	Bundle ID	com.automattic.gravatar-sdk-demo-uikit.prototype-build
	Commit	89a92ce
	App Center Build	Gravatar SDK Demo - UIKit #576

Automatticians: You can use our internal self-serve MC tool to give yourself access to App Center if needed.

[andrewdmontgomery]

This branch currently contains:

• a temporarily modified Package.swift
• Both Package.resolved files UNmodified

So the Dangerfile check should fail. And it does:

Note that only one error is returned. Danger appears to be deduplicating errors. I have confirmed locally that each function call causes the warning (by commenting out the other function call).

[andrewdmontgomery]

I just implemented the install_swiftpm_dependencies plugin, and would you look at this:

xcodebuild: error: Could not resolve package dependencies:
  an out-of-date resolved file was detected at /opt/ci/builds/builder/automattic/gravatar-sdk-ios/Demo/Gravatar-Demo.xcodeproj/project.xcworkspace/xcshareddata/swiftpm/Package.resolved, which is not allowed when automatic dependency resolution is disabled; please make sure to update the file to reflect the changes in dependencies. Running resolver because the following dependencies were added: 'swiftformat' (https://github.com/nicklockwood/SwiftFormat)

And it fails the build, too.

Let's keep the Dangerfile check. The output is easier to find.

[andrewdmontgomery]

Placing this in Milestone 6, since it shouldn't block the release. I'm still targeting the release branch, though, in case we can merge it before we release.

[AliSoftware]