Unify all stage2 tools into a workspace

This avoids double compiled Cargo. Hopefully this would speed up (extended) compilation for ~10m.

Notes: when updating Cargo submodule, the replacement version may also need to be updated.

Author: ishitatsuyuki

.gitmodules

@@ -23,7 +23,7 @@
 	url = https://github.com/rust-lang-nursery/nomicon.git
 [submodule "src/tools/cargo"]
 	path = src/tools/cargo
-	url = https://github.com/rust-lang/cargo
+	url = https://github.com/rust-lang/cargo.git
 [submodule "reference"]
 	path = src/doc/reference
 	url = https://github.com/rust-lang-nursery/reference.git
@@ -32,4 +32,4 @@
 	url = https://github.com/rust-lang/book.git
 [submodule "src/tools/rls"]
 	path = src/tools/rls
-	url = https://github.com/rust-lang-nursery/rls
+	url = https://github.com/rust-lang-nursery/rls.git