Apply the same manifest attributes to the sources jar. Including a list of client only entries.

Will be used to split the sources into client/common jars in a later change.

src/main/java/net/fabricmc/loom/task/RemapTaskConfiguration.java

@@ -128,6 +128,7 @@ public class RemapTaskConfiguration {
 
 			task.dependsOn(sourcesJarTask);
 			task.getInputFile().convention(sourcesJarTask.getArchiveFile());
+			task.getIncludesClientOnlyClasses().set(project.provider(extension::areEnvironmentSourceSetsSplit));
 		});
 
 		tasks.named(BasePlugin.ASSEMBLE_TASK_NAME).configure(task -> task.dependsOn(remapSourcesTask));