[wip] Update boostrap tests to support book dependencies

Since TRPL now depends on a `trpl` crate, the test needs to be able to
build that crate to run mdbook against it, and also to invoke mdbook
with `--library-path` in that case.

Still to-do:

- [ ] Validate the correct argument form for `--library-path`.
- [ ] Test this to make sure it works (!) and that it does not generate
      any extra noise during build.

src/bootstrap/src/core/build_steps/test.rs

@@ -2171,6 +2171,7 @@ struct BookTest {
     path: PathBuf,
     name: &'static str,
     is_ext_doc: bool,
+    cli_args: Vec<&'static str>,
 }
 
 impl Step for BookTest {
@@ -2223,6 +2224,11 @@ impl BookTest {
         // Books often have feature-gated example text.
         rustbook_cmd.env("RUSTC_BOOTSTRAP", "1");
         rustbook_cmd.env("PATH", new_path).arg("test").arg(path);
+
+        if !self.cli_args.is_empty() {
+            rustbook_cmd.args(self.cli_args);
+        }
+
         builder.add_rust_test_threads(&mut rustbook_cmd);
         let _guard = builder.msg(
             Kind::Test,
@@ -2281,6 +2287,7 @@ macro_rules! test_book {
         $name:ident, $path:expr, $book_name:expr,
         default=$default:expr
         $(,submodules = $submodules:expr)?
+        $(,dependencies=$dependencies:expr)?
         ;
     )+) => {
         $(
@@ -2310,11 +2317,33 @@ macro_rules! test_book {
                             builder.require_submodule(submodule, None);
                         }
                     )*
+
+                    let dependencies = vec![];
+                    let cli_args = vec![];
+                    $(
+                        let mut dependencies = dependencies;
+                        let mut cli_args = cli_args;
+                        for dep in $dependencies {
+                            dependencies.push(dep.into_string());
+                        }
+
+                        if !dependencies.is_empty() {
+                            cli_args.push("--library-path");
+                            cli_args.extend(
+                                dependencies
+                                    .iter()
+                                    .map(|dep| format!("{dep}/target/debug/deps"))
+                                    .collect()
+                            );
+                        }
+                    )?
+
                     builder.ensure(BookTest {
                         compiler: self.compiler,
                         path: PathBuf::from($path),
                         name: $book_name,
                         is_ext_doc: !$default,
+                        cli_args,
                     });
                 }
             }
@@ -2329,7 +2358,7 @@ test_book!(
     RustcBook, "src/doc/rustc", "rustc", default=true;
     RustByExample, "src/doc/rust-by-example", "rust-by-example", default=false, submodules=["src/doc/rust-by-example"];
     EmbeddedBook, "src/doc/embedded-book", "embedded-book", default=false, submodules=["src/doc/embedded-book"];
-    TheBook, "src/doc/book", "book", default=false, submodules=["src/doc/book"];
+    TheBook, "src/doc/book", "book", default=false, submodules=["src/doc/book"], dependencies=["src/doc/book/packages/trpl"];
     UnstableBook, "src/doc/unstable-book", "unstable-book", default=true;
     EditionGuide, "src/doc/edition-guide", "edition-guide", default=false, submodules=["src/doc/edition-guide"];
 );