Add log message to MCP listLibraries step

Closes #94.

Hopefully people won't be as confused about
why it takes so long now.

src/main/java/net/fabricmc/loom/configuration/providers/forge/mcpconfig/StepLogic.java

@@ -171,6 +171,7 @@ public interface StepLogic {
 	final class ListLibraries implements StepLogic {
 		@Override
 		public void execute(ExecutionContext context) throws IOException {
+			context.logger().lifecycle(":downloading minecraft libraries, this may take a while...");
 			try (PrintWriter writer = new PrintWriter(Files.newBufferedWriter(context.setOutput("libraries.txt")))) {
 				for (File lib : context.getMinecraftLibraries()) {
 					writer.println("-e=" + lib.getAbsolutePath());