refactor(netx): reorganize by topic (#800)

Before finishing the ongoing refactoring and leaving whatever
is left of netx in tree, I would like to restructure it so that
we'll have an easy time next time we need to modify it.

Currently, every functionality lives into the `netx.go` file and
we have a support file called `httptransport.go`.

I would like to reorganize by topic, instead. This would allow
future me to more easily perform topic-specific changes.

While there, improve `netx`'s documentation and duplicate some of
this documentation inside `internal/README.md` to provide pointers
to previous documentation, historical context, and some help to
understand the logic architecture of network extensions (aka `netx`).

Part of https://github.com/ooni/probe-cli/pull/396

internal/engine/netx/resolver_test.go

@@ -0,0 +1,33 @@
+package netx
+
+import (
+	"context"
+	"errors"
+	"testing"
+
+	"github.com/apex/log"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+	"github.com/ooni/probe-cli/v3/internal/tracex"
+)
+
+func TestNewResolverBogonResolutionNotBroken(t *testing.T) {
+	saver := new(tracex.Saver)
+	r := NewResolver(Config{
+		BogonIsError: true,
+		DNSCache: map[string][]string{
+			"www.google.com": {"127.0.0.1"},
+		},
+		Saver:  saver,
+		Logger: log.Log,
+	})
+	addrs, err := r.LookupHost(context.Background(), "www.google.com")
+	if !errors.Is(err, netxlite.ErrDNSBogon) {
+		t.Fatal("not the error we expected")
+	}
+	if err.Error() != netxlite.FailureDNSBogonError {
+		t.Fatal("error not correctly wrapped")
+	}
+	if len(addrs) > 0 {
+		t.Fatal("expected no addresses here")
+	}
+}