Allow plugins to contribute BuiltinMacros

[owenv]

Currently, all of the builtin macros used by SwiftBuild are centralized in BuiltinMacros.swift. We should update the build settings initialization process to allow defining these in plugins as well, so that macros associated with a feature can live in the plugin which implements this feature.

[jakepetroules]

This might require some pretty extensive refactoring given that all of the builtin macros are statically initialized as part of the builtin macro namespace. But I think it's also one of the last major pieces of static data in the codebase, so will be really nice to get this taken care of!