Vendor trpl crate so The Book tests work offline

Without this change:

    $ ./x test --set build.vendor=true src/doc/book
    # (lots of output)
    error: failed to select a version for the requirement `futures = "^0.3"` (locked to 0.3.30)
    candidate versions found which didn't match: 0.3.31, 0.3.27
    location searched: directory source `/Users/chris/dev/rust-lang/rust/vendor` (which is replacing registry `crates-io`)
    required by package `trpl v0.2.0 (/Users/chris/dev/rust-lang/rust/src/doc/book/packages/trpl)`
    perhaps a crate was updated and forgotten to be re-vendored?
    Build completed unsuccessfully in 0:01:19

With this change:

    $ ./x test --set build.vendor=true src/doc/book
    # (lots of build output)
    Testing stage1 mdbook src/doc/book (aarch64-apple-darwin)
        finished in 86.949 seconds
    Build completed successfully in 0:04:05

# Conflicts:
#	src/bootstrap/src/core/build_steps/vendor.rs

Author: chriskrycho

src/bootstrap/src/core/build_steps/vendor.rs

@@ -20,6 +20,7 @@ pub fn default_paths_to_vendor(builder: &Builder<'_>) -> Vec<(PathBuf, Vec<&'sta
         ("src/tools/rustbook/Cargo.toml", SUBMODULES_FOR_RUSTBOOK.into()),
         ("src/tools/rustc-perf/Cargo.toml", vec!["src/tools/rustc-perf"]),
         ("src/tools/opt-dist/Cargo.toml", vec![]),
+        ("src/doc/book/packages/trpl/Cargo.toml", vec![]),
     ]
     .into_iter()
     .map(|(path, submodules)| (builder.src.join(path), submodules))