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/dialer.go

@@ -0,0 +1,26 @@
+package netx
+
+//
+// Dialer from Config.
+//
+
+import (
+	"github.com/ooni/probe-cli/v3/internal/bytecounter"
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+// NewDialer creates a new Dialer from the specified config.
+func NewDialer(config Config) model.Dialer {
+	if config.FullResolver == nil {
+		config.FullResolver = NewResolver(config)
+	}
+	logger := model.ValidLoggerOrDefault(config.Logger)
+	d := netxlite.NewDialerWithResolver(
+		logger, config.FullResolver, config.Saver.NewConnectObserver(),
+		config.ReadWriteSaver.NewReadWriteObserver(),
+	)
+	d = netxlite.NewMaybeProxyDialer(d, config.ProxyURL)
+	d = bytecounter.MaybeWrapWithContextAwareDialer(config.ContextByteCounting, d)
+	return d
+}