Only load compiler plugins from direct dependencies

[markvasiv]

What

• Change plugin collection in compile() to walk only the direct modules of the compilation's swift_infos and private_swift_infos, making the code match its documented intent ("we only need to load plugins directly used by the module being compiled"). Module propagation (search paths, explicit module maps) is untouched, so dependents can still resolve the serialized macro attribute when deserializing.
• Behavior change: a module that uses a macro while depending on the interface library only transitively (e.g. through an @_exported re-export) used to compile by accident, because the plugin arrived transitively. It now fails with a clear "macro implementation not found" error until the direct dependency is declared. This matches SwiftPM, which passes macro plugins only to targets that declare the macro dependency.
• Add test/fixtures/macros and a macro_plugin analysis-test suite verifying that the declaring module and its direct dependent get -load-plugin-executable, and the transitive dependent does not.

Why

• With the change, editing a macro implementation only recompiles the modules that actually load the plugin. Measured on a large app where a macro interface library sits below a foundational module (~2,200 transitive dependents), an implementation edit drops from 2,224 actions (~9.5 min) to 7 actions (~12 s).
• Fixes Compiler plugins are loaded by all transitive dependents, making the plugin executable an input to every downstream Swift compile #1884.