Virtual std module subdirectories (#14040)

# Description

Uses "normal" module `std/<submodule>/mod.nu` instead of renaming the
files (as requested in #13842).

# User-Facing Changes

No user-facing changes other than in `view files` results. Imports
remain the same after this PR.

# Tests + Formatting

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

Also manually confirmed that it does not interfere with nupm, since we
did have a conflict at one point (and it's not possible to test here).

# Performance Tests

## Linux

### Nushell Startup - No config

```nu
bench --pretty -n 200  { <path_to>/nu -c "exit" }
```

| Release | Startup Time |
| --- | --- |
| 0.98.0 | 22ms 730µs 768ns +/- 1ms 515µs 942ns
| This commit | 9ms 312µs 68ns +/- 709µs 378ns
| Yesterday's nightly | 9ms 230µs 953ns +/- 9ms 67µs 689ns

### Nushell Startup - Load full standard library

Measures relative impact of a full `use std *`, which isn't recommended,
but worth tracking.

```nu
bench --pretty -n 200  { <path_to>/nu -c "use std *; exit" }
```

| Release | Startup Time |
| --- | --- |
| 0.98.0 | 23ms 10µs 636ns +/- 1ms 277µs 854ns
| This commit | 26ms 922µs 769ns +/- 562µs 538ns
| Yesterday's nightly | 28ms 133µs 95ns +/- 761µs 943ns
| `deprecated_dirs` removal PR * | 23ms 610µs 333ns +/- 369µs 436ns

\* Current increase is partially due to double-loading `dirs` with
removal warning in older version.

# After Submitting

Still TODO - Update standard library doc

crates/nu-std/std/deprecated_dirs/mod.nu

@@ -0,0 +1,161 @@
+# Maintain a list of working directories and navigate them
+
+# The directory stack.
+#
+# Exception: the entry for the current directory contains an
+# irrelevant value. Instead, the source of truth for the working
+# directory is $env.PWD. It has to be this way because cd doesn't
+# know about this module.
+#
+# Example: the following state represents a user-facing directory
+# stack of [/a, /var/tmp, /c], and we are currently in /var/tmp .
+#
+#     PWD = /var/tmp
+#     DIRS_POSITION = 1
+#     DIRS_LIST = [/a, /b, /c]
+#
+# This situation could arise if we started with [/a, /b, /c], then
+# we changed directories from /b to /var/tmp.
+export-env {
+    $env.DIRS_POSITION = 0
+    $env.DIRS_LIST = [($env.PWD | path expand)]
+}
+
+def deprecation_warning [ ] {
+print -e $"
+(ansi red)Warning:(ansi reset) The 'std dirs' module will no longer automatically
+be loaded in the next release. To continue using the Shells
+feature, and to remove this warning, please add the following
+to your startup configuration \(typically env.nu or config.nu\):
+
+use std/dirs shells-aliases *
+
+Or see the documentation for more options.
+"
+}
+
+# Add one or more directories to the list.
+# PWD becomes first of the newly added directories.
+export def --env add [
+    ...paths: string    # directory or directories to add to working list
+] {
+    deprecation_warning
+
+    mut abspaths = []
+    for p in $paths {
+        let exp = ($p | path expand)
+        if ($exp | path type) != 'dir' {
+            let span = (metadata $p).span
+            error make {msg: "not a directory", label: {text: "not a directory", span: $span } }
+        }
+        $abspaths = ($abspaths | append $exp)
+    }
+
+    $env.DIRS_LIST = ($env.DIRS_LIST | insert ($env.DIRS_POSITION + 1) $abspaths | flatten)
+
+    _fetch 1
+}
+
+export alias enter = add
+
+# Advance to the next directory in the list or wrap to beginning.
+export def --env next [
+    N:int = 1   # number of positions to move.
+] {
+    deprecation_warning
+    _fetch $N
+}
+
+export alias n = next
+
+# Back up to the previous directory or wrap to the end.
+export def --env prev [
+    N:int = 1   # number of positions to move.
+] {
+    deprecation_warning
+    _fetch (-1 * $N)
+}
+
+export alias p = prev
+
+# Drop the current directory from the list, if it's not the only one.
+# PWD becomes the next working directory
+export def --env drop [] {
+    deprecation_warning
+    if ($env.DIRS_LIST | length) > 1 {
+        $env.DIRS_LIST = ($env.DIRS_LIST | reject $env.DIRS_POSITION)
+        if ($env.DIRS_POSITION >= ($env.DIRS_LIST | length)) {$env.DIRS_POSITION = 0}
+    }
+
+    # step to previous slot
+    _fetch -1 --forget_current --always_cd
+
+}
+
+export alias dexit = drop
+
+# Display current working directories.
+export def --env show [] {
+    deprecation_warning
+    mut out = []
+    for $p in ($env.DIRS_LIST | enumerate) {
+        let is_act_slot = $p.index == $env.DIRS_POSITION
+        $out = ($out | append [
+            [active, path];
+            [($is_act_slot),
+            (if $is_act_slot {$env.PWD} else {$p.item})   # show current PWD in lieu of active slot
+            ]
+        ])
+    }
+
+    $out
+}
+
+export alias shells = show
+
+export def --env goto [shell?: int] {
+    deprecation_warning
+    if $shell == null {
+        return (show)
+    }
+
+    if $shell < 0 or $shell >= ($env.DIRS_LIST | length) {
+        let span = (metadata $shell | get span)
+        error make {
+            msg: $"(ansi red_bold)invalid_shell_index(ansi reset)"
+            label: {
+                text: $"`shell` should be between 0 and (($env.DIRS_LIST | length) - 1)"
+                span: $span
+            }
+        }
+    }
+
+    _fetch ($shell - $env.DIRS_POSITION)
+}
+
+export alias g = goto
+
+# fetch item helper
+def --env _fetch [
+    offset: int,        # signed change to position
+    --forget_current    # true to skip saving PWD
+    --always_cd         # true to always cd
+] {
+    if not ($forget_current) {
+        # first record current working dir in current slot of ring, to track what CD may have done.
+        $env.DIRS_LIST = ($env.DIRS_LIST | upsert $env.DIRS_POSITION $env.PWD)
+    }
+
+    # figure out which entry to move to
+    # nushell 'mod' operator is really 'remainder', can return negative values.
+    # see: https://stackoverflow.com/questions/13683563/whats-the-difference-between-mod-and-remainder
+    let len = ($env.DIRS_LIST | length)
+    mut pos = ($env.DIRS_POSITION + $offset) mod $len
+    if ($pos < 0) { $pos += $len}
+
+    # if using a different position in ring, CD there.
+    if ($always_cd or $pos != $env.DIRS_POSITION) {
+        $env.DIRS_POSITION = $pos
+        cd ($env.DIRS_LIST | get $pos )
+    }
+}