disable directory submodule auto export (#11157)

should
- close https://github.com/nushell/nushell/issues/11133

# Description
to allow more freedom when writing complex modules, we are disabling the
auto-export of director modules.

the change was as simple as removing the crawling of files and modules
next to any `mod.nu` and update the standard library.

# User-Facing Changes
users will have to explicitely use `export module <mod>` to define
submodules and `export use <mod> <cmd>` to re-export definitions, e.g.
```nushell
# my-module/mod.nu
export module foo.nu     # export a submodule
export use bar.nu bar-1  # re-export an internal command

export def top [] {
    print "`top` from `mod.nu`"
}
```
```nushell
# my-module/foo.nu
export def "foo-1" [] {
    print "`foo-1` from `lib/foo.nu`"
}

export def "foo-2" [] {
    print "`foo-2` from `lib/foo.nu`"
}
```
```nushell
# my-module/bar.nu
export def "bar-1" [] {
    print "`bar-1` from `lib/bar.nu`"
}
```

# Tests + Formatting
i had to add `export module` calls in the `tests/modules/samples/spam`
directory module and allow the `not_allowed` module to not give an
error, it is just empty, which is fine.

# After Submitting
- mention in the release note
- update the following repos
```
#┬─────name─────┬version┬─type─┬─────────repo─────────
0│nu-git-manager│0.4.0  │module│amtoine/nu-git-manager
1│nu-scripts    │0.1.0  │module│amtoine/scripts       
2│nu-zellij     │0.1.0  │module│amtoine/zellij-layouts
3│nu-scripts    │0.1.0  │module│nushell/nu_scripts    
4│nupm          │0.1.0  │module│nushell/nupm          
─┴──────────────┴───────┴──────┴──────────────────────
```

crates/nu-std/std/mod.nu

@@ -1,12 +1,22 @@
 # std.nu, `used` to load all standard library components
 
+export module assert.nu
+export module dirs.nu
+export module dt.nu
+export module formats.nu
+export module help.nu
+export module input.nu
+export module iter.nu
+export module log.nu
+export module math.nu
+export module testing.nu
+export module xml.nu
 export-env {
     use dirs.nu []
     use log.nu []
 }
 
 use dt.nu [datetime-diff, pretty-print-duration]
-use log.nu
 
 # Add the given paths to the PATH.
 #