Method flatten

src/main/java/com/diffplug/gradle/FileMisc.java:301–316

Flattens a single directory (moves its children to be its peers, then deletes the given directory. ``` before: root/ toFlatten/ child1 child2 flatten("root/toFlatten") after: root/ child1 child2 ```

(File dirToRemove)

	 * ```
	 */
	public static void flatten(File dirToRemove) throws IOException {
		final File parent = dirToRemove.getParentFile();
		// move each child directory to the parent
		for (File child : FileMisc.list(dirToRemove)) {
			boolean createDestDir = false;
			if (child.isFile()) {
				FileUtils.moveFileToDirectory(child, parent, createDestDir);
			} else if (child.isDirectory()) {
				FileUtils.moveDirectoryToDirectory(child, parent, createDestDir);
			} else {
				throw new IllegalArgumentException("Unknown filetype: " + child);
			}
		}
		// remove the directory which we're flattening away
		FileMisc.forceDelete(dirToRemove);
	}

	/** Concats the first files and writes them to the last file. */
	public static void concat(Iterable<File> toMerge, File dst) throws IOException {