go: create better endpoint for packages directory

The [go-modules](https://github.com/bytecodealliance/go-modules) project uses an endpoint for their wit types that looks like this: `go.bytecodealliance.org/cm`. I'm wondering if we can create a similar endpoint for wit-bindgen's Go package and have it be something like `go.bytecodealliance.org/wit-bindgen`. This would mean we wouldn't need to add a replace directive in the mod file:

```diff
require (
- github.com/bytecodealliance/wit-bindgen v0.0.0-00010101000000-000000000000
+ go.bytecodealliance.org/wit-bindgen v0.51.0
github.com/julienschmidt/httprouter v1.3.0
)

- replace github.com/bytecodealliance/wit-bindgen => github.com/bytecodealliance/wit-bindgen/crates/go/src/package v0.51.0
```

Also, I don't have super strong opinions on what the actual endpoint will be named. If there are other endpoints that folks like better than `/wit-bindgen`, please feel free to share!