[Macros] Plugin search options group

'load-plugin-library', 'load-plugin-executable', '-plugin-path' and
'-external-plugin-path' should be searched in the order they are
specified in the arguments.

Previously, for example '-plugin-path' used to precede
'-external-plugin-path' regardless of the position in the arguments.

lib/Serialization/SerializedModuleLoader.cpp

@@ -205,8 +205,7 @@ void SerializedModuleLoaderBase::collectVisibleTopLevelModuleNamesImpl(
       });
       return None;
     }
-    case ModuleSearchPathKind::RuntimeLibrary:
-    case ModuleSearchPathKind::CompilerPlugin: {
+    case ModuleSearchPathKind::RuntimeLibrary: {
       // Look for:
       // (Darwin OS) $PATH/{name}.swiftmodule/{arch}.{extension}
       // (Other OS)  $PATH/{name}.{extension}
@@ -678,8 +677,7 @@ bool SerializedModuleLoaderBase::findModule(
 
     switch (searchPath->getKind()) {
     case ModuleSearchPathKind::Import:
-    case ModuleSearchPathKind::RuntimeLibrary:
-    case ModuleSearchPathKind::CompilerPlugin: {
+    case ModuleSearchPathKind::RuntimeLibrary: {
       isFramework = false;
 
       // On Apple platforms, we can assume that the runtime libraries use