Simplify Webpack References by encoding file path + export name as single id (#26300)

We always look up these references in a map so it doesn't matter what
their value is. It could be a hash for example.

The loaders now encode a single $$id instead of filepath + name.

This changes the react-client-manifest to have a single level. The value
inside the map is still split into module id + export name because
that's what gets looked up in webpack.

The react-ssr-manifest is still two levels because that's a reverse
lookup.