Begin refactor target dependency resolution to be more easily extensible

[owenv]

This is easiest to review one commit at a time. It's a series of refactors to fix some small correctness issues in target dependency resolution and make it more easily extensible as I get ready to integrate artifact bundle support.

1. Ensure most clients are fetching settings for a target using the GlobalProductPlan instead of the BuildRequestContext + TargetDependencyGraph. This ensures imparted build properties and anything we add to the global product plan in the future are correctly taken into account
2. Remove the unused CapturedBuildInfo mechanism, which relied on direct access to the TargetBuildGraph
3. Fix some issues with how the symbol extraction support looked up dependency settings using the build graph
4. Reduce some API surface of the TargetBuildGraph to encourage use of the GlobalProductPlan instead
5. Refactor GlobalProductPlan into discrete mostly-functional passes to work towards improved parallelization, caching, and separation of responsibilities

[owenv]

@swift-ci test

[owenv]

@swift-ci test

[owenv]

@swift-ci test

[owenv]

@swift-ci test