Rollup merge of #81223 - GuillaumeGomez:generate-redirect-map, r=jyn514

[rustdoc] Generate redirect map file

Fixes #81134.

So with this code:

```rust
#![crate_name = "foo"]

pub use private::Quz;
pub use hidden::Bar;

mod private {
    pub struct Quz;
}

#[doc(hidden)]
pub mod hidden {
    pub struct Bar;
}

#[macro_export]
macro_rules! foo {
() => {}
}
```

It generates:

```json
{
  "foo/macro.foo!.html": "foo/macro.foo.html",
  "foo/private/struct.Quz.html": "foo/struct.Quz.html",
  "foo/hidden/struct.Bar.html": "foo/struct.Bar.html"
}
```

Do the pathes look as you expected ````@pietroalbini?````

r? ````@jyn514````

src/librustdoc/config.rs

@@ -263,6 +263,8 @@ crate struct RenderOptions {
     crate document_private: bool,
     /// Document items that have `doc(hidden)`.
     crate document_hidden: bool,
+    /// If `true`, generate a JSON file in the crate folder instead of HTML redirection files.
+    crate generate_redirect_map: bool,
     crate unstable_features: rustc_feature::UnstableFeatures,
 }
 
@@ -570,6 +572,7 @@ impl Options {
         let document_private = matches.opt_present("document-private-items");
         let document_hidden = matches.opt_present("document-hidden-items");
         let run_check = matches.opt_present("check");
+        let generate_redirect_map = matches.opt_present("generate-redirect-map");
 
         let (lint_opts, describe_lints, lint_cap) = get_cmd_lint_options(matches, error_format);
 
@@ -627,6 +630,7 @@ impl Options {
                 generate_search_filter,
                 document_private,
                 document_hidden,
+                generate_redirect_map,
                 unstable_features: rustc_feature::UnstableFeatures::from_environment(
                     crate_name.as_deref(),
                 ),