bootstrap.py handling of submodules was removed in
rust-lang#97513.

This is not used anywhere outside this module.

This felt like an important point to me.

The argument was not necessary, since it was only ever passed one
value that exists in the config itself.

Although its origins were in bootstrap.py, that code in bootstrap.py
no longer exists since it was removed.

I put this submodule update in the entirely wrong location. I put it in
the `RustcBook` step (for generating src/doc/rustc), when it really
should exist for all steps that use the `Rustbook` tool.

This adds a new method `require_and_update_submodule` to replace
`update_submodule`. This new method will generate an error if the
submodule doesn't actually exist. This replaces some ad-hoc checks that
were performing this function. This helps ensure that a good error
message is always displayed.

This also adds require_and_update_all_submodules which does this for
all submodules.

Ideally this should not have any change other than better error messages
when submodules are missing.

If the submodule is not checked out, then these tests would fail.

This updates the reference which is now using a new mdbook plugin. This
requires a little extra work than a normal book because the plugin uses
`rustdoc` to generate links to the standard library. It also ensures
that the submodule is available for *any* command that uses rustbook,
since it is now part of the rustbook workspace.

This was a copy/paste mistake.

This makes it easier to call these functions without needing to form a
Path.

…ired

These are required 100% of the time, but they are almost always required
for any command that runs Cargo in the main workspace.

Ideally, initializing these two standard library submodules would be
lazy and only initialized when required (see
rust-lang#82653). However, it would require
updating these in almost every Step (anything that runs `cargo` in the
main workspace).

I misread this one. It is only checking if LLVM needs to be rebuilt.
There is code below that handles the case where it is unable to compute
the stamp if the source is missing.

Just trying to be a little less verbose here.