fix(Node buildpack): Ensure layers are handled in correct order
fix(Node buildpack): Ensure layers are handled in correct order

Also refactors `npm` layer to `node_modules` layer to better reflect semantics.

stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila,stencila/stencila