restructure this multi-crate project to use Cargo's "workspace" feature

this solves an issue in the Nix build, where managing multiple
Cargo.lock files is otherwise tricky. it causes (or fails to fix?) an adjacent issue where
the spirv builder doesn't seem to have everything it needs vendored.

crates/spirv_backend_lib/src

@@ -0,0 +1 @@
+../spirv_backend/src/