Copy A Directory Recursively In Rust Without std::fs::copy

```cargo
[dependencies]
walkdir = "2.5.0"
```

use std::fs;
use std::path::PathBuf;
use walkdir::WalkDir;

fn main() {
  let source = PathBuf::from("target_source");
  let dest = PathBuf::from("target_dest");
  match copy_dir(&source, &dest) {
    Ok(_) => { println!("copied the directory"); }
    Err(e) =>  { println!("{:?}", e); }
  }
}

fn copy_dir(source_dir: &PathBuf, dest_dir: &PathBuf) -> Result<(), std::io::Error> {
  for entry in WalkDir::new(source_dir) {
    let source_path = entry?.into_path();
    let dest_path = dest_dir.join(source_path.strip_prefix(source_dir).unwrap());
    if source_path.is_dir() {
      fs::create_dir_all(dest_path)?;
    }
    else {
      let data = std::fs::read(source_path)?;
      std::fs::write(dest_path, &data)?;
    }
  }
  Ok(())
}