Auto merge of #14557 - shannmu:_cargo_cmds, r=epage

feat: Add custom completer for `cargo help <TAB>`

### What does this PR try to resolve?
Tracking issue #14520

Add custom completer for `cargo help <TAB>`

### Additional information
The current completer function is quite slow because it executes too many functions. One idea I have is to use the file!() macro to list the filenames under the commands directory, excluding mod.rs, and return them as the completion results. Would this approach be too hacky?

Author: bors

src/bin/cargo/commands/help.rs

@@ -15,7 +15,19 @@ const COMPRESSED_MAN: &[u8] = include_bytes!(concat!(env!("OUT_DIR"), "/man.tgz"
 pub fn cli() -> Command {
     subcommand("help")
         .about("Displays help for a cargo subcommand")
-        .arg(Arg::new("COMMAND").action(ArgAction::Set))
+        .arg(Arg::new("COMMAND").action(ArgAction::Set).add(
+            clap_complete::ArgValueCandidates::new(|| {
+                super::builtin()
+                    .iter()
+                    .map(|cmd| {
+                        let name = cmd.get_name();
+                        clap_complete::CompletionCandidate::new(name)
+                            .help(cmd.get_about().cloned())
+                            .hide(cmd.is_hide_set())
+                    })
+                    .collect()
+            }),
+        ))
 }
 
 pub fn exec(gctx: &mut GlobalContext, args: &ArgMatches) -> CliResult {