go: enable user to configure the package name for generated types

[asteurer]

It would be really nice to be able to run a command like wit-bindgen go --pkg github.com/foo/bar/v1 and have the generated types prepend the custom package to the wit_component path:

import "github.com/foo/bar/v1/wit_component/wasi_http_0_2_0_types"

As part of this, we will probably want to alter the wit_bindings.go file:

// Instead of "package main"
package wit_component

This would be helpful for cases where an SDK wants to make use of the bindings, but doesn't want to expose the bindings to a user (see this version of the spin-go-sdk as an example).

If there's a better way to do this, I'm also open to other suggestions.

@dicej if this is something you would like to move forward with, will you assign this to me, please?

[dicej]

Sounds reasonable to me 👍