Don't put Cargo into the rustc workspace

This causes problems when first cloning and bootstrapping the repository
unfortunately, so let's ensure that Cargo sticks around in its own workspace.
Because Cargo is a submodule it's not available by default on the inital clone
of the rust-lang/rust repository. Normally it's the responsibility of the
rustbuild to take care of this, but unfortunately to build rustbuild itself we
need to resolve the workspace conflicts.

To deal with this we'll just have to ensure that all submodules are in their own
workspace, which sort of makes sense anyway as updates to dependencies as
bugfixes to Cargo should go to rust-lang/cargo instead of rust-lang/rust. In any
case this commit removes Cargo from the global workspace which should resolve
the issues that we've been seeing.

To actually perform this the `cargo` submodule has been moved to the top
directory to ensure it's outside the scope of `src/Cargo.toml` as a workspace.

.gitmodules

@@ -22,7 +22,7 @@
 	path = src/doc/nomicon
 	url = https://github.com/rust-lang-nursery/nomicon
 [submodule "src/tools/cargo"]
-	path = src/tools/cargo
+	path = cargo
 	url = https://github.com/rust-lang/cargo
 [submodule "reference"]
 	path = src/doc/reference