Add opt-in direct compiler plugin loading

[markvasiv]

What

• Adds an opt-in swift.load_plugins_from_direct_dependencies feature. When enabled, plugin collection in compile() walks only the direct modules of the compilation's swift_infos and private_swift_infos.
• Transitive plugin loading remains the default behavior.
• Documents the feature on the plugins attribute.
• Adds test/fixtures/macros and a macro_plugin analysis-test suite verifying that:
  • Transitive dependents load the plugin by default.
  • Transitive dependents don't load the plugin if the feature is enabled

Why

• Compiler plugin executables are currently inputs to every transitive dependent's Swift compilation action. As a result, changing a macro implementation invalidates the entire reverse-dependency closure which expensive for large dependency chains.
• Keeping this opt-in preserves compatibility with code that relies on transitive plugin loading.
• Resolves Compiler plugins are loaded by all transitive dependents, making the plugin executable an input to every downstream Swift compile #1884.