refactor: merge dnsx and errorsx into netxlite (#517)

When preparing a tutorial for netxlite, I figured it is easier
to tell people "hey, this is the package you should use for all
low-level networking stuff" rather than introducing people to
a set of packages working together where some piece of functionality
is here and some other piece is there.

Part of https://github.com/ooni/probe/issues/1591

internal/netxlite/dnsoverhttps.go

@@ -0,0 +1,88 @@
+package netxlite
+
+import (
+	"bytes"
+	"context"
+	"errors"
+	"net/http"
+	"time"
+
+	"github.com/ooni/probe-cli/v3/internal/engine/httpheader"
+)
+
+// HTTPClient is the HTTP client expected by DNSOverHTTPS.
+type HTTPClient interface {
+	Do(req *http.Request) (*http.Response, error)
+	CloseIdleConnections()
+}
+
+// DNSOverHTTPS is a DNS over HTTPS RoundTripper. Requests are submitted over
+// an HTTP/HTTPS channel provided by URL using the Do function.
+type DNSOverHTTPS struct {
+	Client       HTTPClient
+	URL          string
+	HostOverride string
+}
+
+// NewDNSOverHTTPS creates a new DNSOverHTTP instance from the
+// specified http.Client and URL, as a convenience.
+func NewDNSOverHTTPS(client HTTPClient, URL string) *DNSOverHTTPS {
+	return NewDNSOverHTTPSWithHostOverride(client, URL, "")
+}
+
+// NewDNSOverHTTPSWithHostOverride is like NewDNSOverHTTPS except that
+// it's creating a resolver where we use the specified host.
+func NewDNSOverHTTPSWithHostOverride(
+	client HTTPClient, URL, hostOverride string) *DNSOverHTTPS {
+	return &DNSOverHTTPS{Client: client, URL: URL, HostOverride: hostOverride}
+}
+
+// RoundTrip implements RoundTripper.RoundTrip.
+func (t *DNSOverHTTPS) RoundTrip(ctx context.Context, query []byte) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(ctx, 45*time.Second)
+	defer cancel()
+	req, err := http.NewRequest("POST", t.URL, bytes.NewReader(query))
+	if err != nil {
+		return nil, err
+	}
+	req.Host = t.HostOverride
+	req.Header.Set("user-agent", httpheader.UserAgent())
+	req.Header.Set("content-type", "application/dns-message")
+	var resp *http.Response
+	resp, err = t.Client.Do(req.WithContext(ctx))
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != 200 {
+		// TODO(bassosimone): we should map the status code to a
+		// proper Error in the DNS context.
+		return nil, errors.New("doh: server returned error")
+	}
+	if resp.Header.Get("content-type") != "application/dns-message" {
+		return nil, errors.New("doh: invalid content-type")
+	}
+	return ReadAllContext(ctx, resp.Body)
+}
+
+// RequiresPadding returns true for DoH according to RFC8467
+func (t *DNSOverHTTPS) RequiresPadding() bool {
+	return true
+}
+
+// Network returns the transport network (e.g., doh, dot)
+func (t *DNSOverHTTPS) Network() string {
+	return "doh"
+}
+
+// Address returns the upstream server address.
+func (t *DNSOverHTTPS) Address() string {
+	return t.URL
+}
+
+// CloseIdleConnections closes idle connections.
+func (t *DNSOverHTTPS) CloseIdleConnections() {
+	t.Client.CloseIdleConnections()
+}
+
+var _ DNSTransport = &DNSOverHTTPS{}