Auto merge of #39917 - alexcrichton:build-cargo, r=brson

rustbuild: Add support for compiling Cargo

This commit adds support to rustbuild for compiling Cargo as part of the release
process. Previously rustbuild would simply download a Cargo snapshot and
repackage it. With this change we should be able to turn off artifacts from the
rust-lang/cargo repository and purely rely on the artifacts Cargo produces here.

The infrastructure added here is intended to be extensible to other components,
such as the RLS. It won't exactly be a one-line addition, but the addition of
Cargo didn't require too much hooplah anyway.

The process for release Cargo will now look like:

* The rust-lang/rust repository has a Cargo submodule which is used to build a
  Cargo to pair with the rust-lang/rust release
* Periodically we'll update the cargo submodule as necessary on rust-lang/rust's
  master branch
* When branching beta we'll create a new branch of Cargo (as we do today), and
  the first commit to the beta branch will be to update the Cargo submodule to
  this exact revision.
* When branching stable, we'll ensure that the Cargo submodule is updated and
  then make a stable release.

Backports to Cargo will look like:

* Send a PR to cargo's master branch
* Send a PR to cargo's release branch (e.g. rust-1.16.0)
* Send a PR to rust-lang/rust's beta branch updating the submodule
* Eventually send a PR to rust-lang/rust's master branch updating the submodule

For reference, the process to add a new component to the rust-lang/rust release
would look like:

* Add `$foo` as a submodule in `src/tools`
* Add a `tool-$foo` step which compiles `$foo` with the specified compiler,
  likely mirroring what Cargo does.
* Add a `dist-$foo` step which uses `src/tools/$foo` and the `tool-$foo` output
  to create a rust-installer package for `$foo` likely mirroring what Cargo
  does.
* Update the `dist-extended` step with a new dependency on `dist-$foo`
* Update `src/tools/build-manifest` for the new component.

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