[opam] Change the pattern for pulling down vendored files

[phated]

There was a really cool pattern I was introduced to in https://github.com/ocaml/opam-repository/pull/17776/files where there can be an opam file that just fetches a prebuilt dependency. We should do that for binaryen instead of shoving them inside this package.

[ospencer]

That'd be sick!

[phated]

This pattern doesn't work for esy so opting for always building from source. Ref #113