improve completions of use and overlay use (#11330)

# Description
this PR is two-fold
- make `use` and `overlay use` use the same completion algorithm in
48f29b633
- list directory modules in completions of both with 402acde5c

# User-Facing Changes
i currently have the following in my `NU_LIB_DIRS`
<details>
<summary>click to see the script</summary>

```nushell
for dir in $env.NU_LIB_DIRS {
    print $dir
    print (ls $dir --short-names | select name type)
}
```
</details>

```
/home/amtoine/.local/share/nupm/modules
#┬────────name────────┬type
0│nu-git-manager      │dir
1│nu-git-manager-sugar│dir
2│nu-hooks            │dir
3│nu-scripts          │dir
4│nu-themes           │dir
5│nupm                │dir
─┴────────────────────┴────

/home/amtoine/.config/nushell/overlays
#┬──name──┬type
0│ocaml.nu│file
─┴────────┴────
```

> **Note**
> all the samples below are run from the Nushell repo, i.e. a directory
with a `toolkit.nu` module

## before the changes
- `use` would give me `["ocaml.nu", "toolkit.nu"]` 
- `overlay use` would give me `[]` 

## after the changes
both commands give me
```nushell
[
    "nupm/",
    "ocaml.nu",
    "toolkit.nu",
    "nu-scripts/",
    "nu-git-manager/",
    "nu-git-manager-sugar/",
]
```

# Tests + Formatting
- adds a new `directory_completion/mod.nu` to the completion fixtures
- make sure `source-env`, `use` and `overlay-use` are all tested in the
_dotnu_ test
- fix all the other tests that use completions in the fixtures directory
for completions

# After Submitting

crates/nu-cli/src/completions/completer.rs

@@ -250,7 +250,9 @@ impl NuCompleter {
                                             working_set.get_span_contents(previous_expr.0).to_vec();
 
                                         // Completion for .nu files
-                                        if prev_expr_str == b"use" || prev_expr_str == b"source-env"
+                                        if prev_expr_str == b"use"
+                                            || prev_expr_str == b"overlay use"
+                                            || prev_expr_str == b"source-env"
                                         {
                                             let mut completer =
                                                 DotNuCompletion::new(self.engine_state.clone());

crates/nu-cli/src/completions/dotnu_completions.rs

@@ -5,7 +5,7 @@ use nu_protocol::{
 };
 use reedline::Suggestion;
 use std::{
-    path::{is_separator, MAIN_SEPARATOR as SEP, MAIN_SEPARATOR_STR},
+    path::{is_separator, Path, MAIN_SEPARATOR as SEP, MAIN_SEPARATOR_STR},
     sync::Arc,
 };
 
@@ -91,16 +91,21 @@ impl Completer for DotNuCompletion {
         // and transform them into suggestions
         let output: Vec<Suggestion> = search_dirs
             .into_iter()
-            .flat_map(|it| {
-                file_path_completion(span, &partial, &it, options)
+            .flat_map(|search_dir| {
+                let completions = file_path_completion(span, &partial, &search_dir, options);
+                completions
                     .into_iter()
-                    .filter(|it| {
+                    .filter(move |it| {
                         // Different base dir, so we list the .nu files or folders
                         if !is_current_folder {
                             it.1.ends_with(".nu") || it.1.ends_with(SEP)
                         } else {
-                            // Lib dirs, so we filter only the .nu files
-                            it.1.ends_with(".nu")
+                            // Lib dirs, so we filter only the .nu files or directory modules
+                            if it.1.ends_with(SEP) {
+                                Path::new(&search_dir).join(&it.1).join("mod.nu").exists()
+                            } else {
+                                it.1.ends_with(".nu")
+                            }
                         }
                     })
                     .map(move |x| Suggestion {