Fix -x option behavior

This PR addresses an issue where `target_dirs` were being optimized out\
before being passed to the `get_filesystem_devices()` function.
The changes in this PR ensure that `target_dirs` are passed directly
to `get_filesystem_devices()`, and only then are they simplified.

Example of a command that currently doesn't work correctly:
```sh
dust -x $(findmnt -S tmpfs -o target -n)
```

It should show the usage of all tmpfs mounts, but it currently shows
just the the root mountpoints like `/run`, since all the mountpoints
nested under it are optimized out.

src/main.rs

@@ -192,10 +192,10 @@ fn main() {
     let limit_filesystem = options.get_flag("limit_filesystem");
     let follow_links = options.get_flag("dereference_links");
 
-    let simplified_dirs = simplify_dir_names(target_dirs);
     let allowed_filesystems = limit_filesystem
-        .then(|| get_filesystem_devices(simplified_dirs.iter()))
+        .then(|| get_filesystem_devices(target_dirs.iter().cloned()))
         .unwrap_or_default();
+    let simplified_dirs = simplify_dir_names(target_dirs);
 
     let ignored_full_path: HashSet<PathBuf> = ignore_directories
         .into_iter()