FIX: correct bad span in std help errors (#9039)

# Description
## ❌ before this PR
```
>_ std help modules euwioq
Error: nu:🐚:external_command

  × External command failed
     ╭─[help:259:1]
 259 │         if ($found_module | is-empty) {
 260 │             module_not_found_error (metadata $module | get span)
     ·                                    ──────────────┬──────────────
     ·                                                  ╰── Cannot convert record<start: int, end: int> to a string
 261 │         }
     ╰────
  help: All arguments to an external command need to be string-compatible
```
```
>_ std help externs euwioq
Error:
  × std::help::extern_not_found
     ╭─[help:401:1]
 401 │
 402 │     let extern = ($extern | str join " ")
     ·     ─┬─
     ·      ╰── extern not found
 403 │
     ╰────
```
> **Note**
> same kind of error with all the others

## ✔️ after this PR
```
> std help modules euwioq                                                                                         04/28/2023 05:45:50 PM
Error:
  × std::help::module_not_found
   ╭─[entry #2:1:1]
 1 │ std help modules euwioq
   ·                  ───┬──
   ·                     ╰── module not found
   ╰────
```
```
> std help externs euwioq                                                                                         04/28/2023 05:45:53 PM
Error:
  × std::help::extern_not_found
   ╭─[entry #3:1:1]
 1 │ std help externs euwioq
   ·                  ───┬──
   ·                     ╰── extern not found
   ╰────
```
> **Note**
> same with the others

# User-Facing Changes
fixes the errors to have proper messages

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`

# After Submitting
```
$nothing
```

crates/nu-std/lib/help.nu

@@ -243,15 +243,13 @@ export def "help modules" [
 ] {
     let modules = $nu.scope.modules
 
-    let module = ($module | str join " ")
-
     if not ($find | is-empty) {
         $modules | find $find --columns [name usage]
     } else if not ($module | is-empty) {
-        let found_module = ($modules | where name == $module)
+        let found_module = ($modules | where name == ($module | str join " "))
 
         if ($found_module | is-empty) {
-            module_not_found_error (metadata $module | get span)
+            module-not-found-error (metadata $module | get span)
         }
 
         show-module ($found_module | get 0)
@@ -347,12 +345,10 @@ export def "help aliases" [
 ] {
     let aliases = ($nu.scope.aliases | sort-by name)
 
-    let alias = ($alias | str join " ")
-
     if not ($find | is-empty) {
         $aliases | find $find --columns [name usage]
     } else if not ($alias | is-empty) {
-        let found_alias = ($aliases | where name == $alias)
+        let found_alias = ($aliases | where name == ($alias | str join " "))
 
         if ($found_alias | is-empty) {
             alias-not-found-error (metadata $alias | get span)
@@ -387,12 +383,10 @@ export def "help externs" [
         | str trim
     )
 
-    let extern = ($extern | str join " ")
-
     if not ($find | is-empty) {
         $externs | find $find --columns [name usage]
     } else if not ($extern | is-empty) {
-        let found_extern = ($externs | where name == $extern)
+        let found_extern = ($externs | where name == ($extern | str join " "))
 
         if ($found_extern | is-empty) {
             extern-not-found-error (metadata $extern | get span)
@@ -453,12 +447,10 @@ export def "help operators" [
 ] {
     let operators = (get-all-operators)
 
-    let operator = ($operator | str join " ")
-
     if not ($find | is-empty) {
         $operators | find $find --columns [type name]
     } else if not ($operator | is-empty) {
-        let found_operator = ($operators | where name == $operator)
+        let found_operator = ($operators | where name == ($operator | str join " "))
 
         if ($found_operator | is-empty) {
             operator-not-found-error (metadata $operator | get span)
@@ -657,18 +649,17 @@ export def "help commands" [
 ] {
     let commands = ($nu.scope.commands | where not is_extern | reject is_extern | sort-by name)
 
-    let command = ($command | str join " ")
-
     if not ($find | is-empty) {
         # TODO: impl find for external commands
         $commands | find $find --columns [name usage search_terms] | select name category usage signatures search_terms
     } else if not ($command | is-empty) {
-        let found_command = ($commands | where name == $command)
+        let target_command = ($command | str join " ")
+        let found_command = ($commands | where name == $target_command)
 
         if ($found_command | is-empty) {
             try {
-                print $"(ansi default_italic)Help pages from external command ($command | pretty-cmd):(ansi reset)"
-                ^($env.NU_HELPER? | default "man") $command
+                print $"(ansi default_italic)Help pages from external command ($target_command | pretty-cmd):(ansi reset)"
+                ^($env.NU_HELPER? | default "man") $target_command
             } catch {
                 command-not-found-error (metadata $command | get span)
             }