Allow one pseudo-JS module to access the ModuleInstance of another module

[na--]

(Extracted from #2226, and somewhat connected to #2258 and #1802)

It would be quite nice if one pseudo-JS module is able to access the modules.Instance of another module. For example, if k6/ws is able to access the module instance of k6/http, though ideally any module (regardless of built-in or extension) should be able to access any other module.

This will allow us to move a lot of the (mostly) HTTP-specific cruft from lib.State to a much more appropriate place:

k6/lib/state.go

Lines 56 to 63 in 922c963

	// Networking equipment.
	Transport http.RoundTripper
	Dialer DialContexter
	CookieJar *cookiejar.Jar
	TLSConfig *tls.Config
	// Rate limits.
	RPSLimit *rate.Limiter

But still allow other parts of the codebase (like k6/ws and k6/grpc) to access the default HTTP transport or cookie jar, when they need to.

[na--]

As a bonus, the lengthy initialization of HTTP-specific things that currently happens during VU init:

k6/js/runner.go

Lines 157 to 224 in 922c963

	var cipherSuites []uint16
	if r.Bundle.Options.TLSCipherSuites != nil {
	cipherSuites = *r.Bundle.Options.TLSCipherSuites
	}
	var tlsVersions lib.TLSVersions
	if r.Bundle.Options.TLSVersion != nil {
	tlsVersions = *r.Bundle.Options.TLSVersion
	}
	tlsAuth := r.Bundle.Options.TLSAuth
	certs := make([]tls.Certificate, len(tlsAuth))
	nameToCert := make(map[string]*tls.Certificate)
	for i, auth := range tlsAuth {
	for _, name := range auth.Domains {
	cert, err := auth.Certificate()
	if err != nil {
	return nil, err
	}
	certs[i] = *cert
	nameToCert[name] = &certs[i]
	}
	}
	dialer := &netext.Dialer{
	Dialer: r.BaseDialer,
	Resolver: r.Resolver,
	Blacklist: r.Bundle.Options.BlacklistIPs,
	BlockedHostnames: r.Bundle.Options.BlockedHostnames.Trie,
	Hosts: r.Bundle.Options.Hosts,
	}
	if r.Bundle.Options.LocalIPs.Valid {
	var ipIndex uint64
	if idLocal > 0 {
	ipIndex = idLocal - 1
	}
	dialer.Dialer.LocalAddr = &net.TCPAddr{IP: r.Bundle.Options.LocalIPs.Pool.GetIP(ipIndex)}
	}
	tlsConfig := &tls.Config{
	InsecureSkipVerify: r.Bundle.Options.InsecureSkipTLSVerify.Bool, //nolint:gosec
	CipherSuites: cipherSuites,
	MinVersion: uint16(tlsVersions.Min),
	MaxVersion: uint16(tlsVersions.Max),
	Certificates: certs,
	NameToCertificate: nameToCert,
	Renegotiation: tls.RenegotiateFreelyAsClient,
	}
	transport := &http.Transport{
	Proxy: http.ProxyFromEnvironment,
	TLSClientConfig: tlsConfig,
	DialContext: dialer.DialContext,
	DisableCompression: true,
	DisableKeepAlives: r.Bundle.Options.NoConnectionReuse.Bool,
	MaxIdleConns: int(r.Bundle.Options.Batch.Int64),
	MaxIdleConnsPerHost: int(r.Bundle.Options.BatchPerHost.Int64),
	}
	if forceHTTP1() {
	transport.TLSNextProto = make(map[string]func(string, *tls.Conn) http.RoundTripper) // send over h1 protocol
	} else {
	_ = http2.ConfigureTransport(transport) // send over h2 protocol
	}
	cookieJar, err := cookiejar.New(nil)
	if err != nil {
	return nil, err
	}

Would be moved to a much more appropriate place in the k6/http module, e.g. here:

k6/js/modules/k6/http/http.go

Lines 69 to 72 in 8a475ab

	mi.defaultClient = &Client{
	// TODO: configure this from lib.Options and get rid of some of the
	// things in the VU State struct that should be here. See
	// https://github.com/grafana/k6/issues/2293