Make the GoGet middleware configurable

[arschles]

You should be able to configure it to redirect to any DNS name or IP - including the proxy IP/name, if running in proxy mode. Right now, it's hard-coded to gomods.io

[bketelsen]

I have some thoughts here, I need to formalize, I think there's more to this.

[arschles]

Continuing conversation on slack, @bketelsen you formalized all (we think) of the possible CLI workflows (copied to here). The question that remains is whether 2.i will cause vgo to hit the http://athensproxywhatever//githublcim/bketelsen/captainhook/@v/... endpoints directly, or still go through the ?go-get=1 flow.

If it's the former, this doesn't matter as much. Otherwise, we need this so the server knows where to redirect

[bketelsen]

here's the code from vgo:

func (p *proxyRepo) Versions(prefix string) ([]string, error) {
	var data []byte
	err := web.Get(p.url+"/@v/list", web.ReadAllBody(&data))
	if err != nil {
		return nil, err
	}

zip, and other implementations are the same, they expect to call the proxy with the url.
also the proxy code has the concept of a base url and path:
gomods.io/proxy could be the "endpoint" for the proxy, which would result in a web request like:
https://gomods.io/proxy/github.com/bketelsen/captainhook/@v/list

[arschles]

thanks @bketelsen - confirms my understanding too. reading the code cemented it too - good idea :)

the GoGet middleware still needs to be configurable for registry use, but not for proxy usage