Recursively copy a directory without specifying every file in it

[Googulator]

Right now, rename supports directories, but copy for some reason doesn't. This means, the only way to copy a directory is by listing every file inside individually in the POM, which is prone to breakage when new files are added.

@kohlschuetter may be interested as well, but https://github.com/kohlschutter/copy-rename-maven-plugin doesn't have issues enabled.