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.go

@@ -0,0 +1,25 @@
+package netx
+
+//
+// Resolver from Config.
+//
+
+import (
+	"github.com/ooni/probe-cli/v3/internal/model"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
+)
+
+// NewResolver creates a new resolver from the specified config.
+func NewResolver(config Config) model.Resolver {
+	if config.BaseResolver == nil {
+		config.BaseResolver = netxlite.NewResolverSystem()
+	}
+	r := netxlite.WrapResolver(
+		model.ValidLoggerOrDefault(config.Logger),
+		config.BaseResolver,
+	)
+	r = netxlite.MaybeWrapWithCachingResolver(config.CacheResolutions, r)
+	r = netxlite.MaybeWrapWithStaticDNSCache(config.DNSCache, r)
+	r = netxlite.MaybeWrapWithBogonResolver(config.BogonIsError, r)
+	return config.Saver.WrapResolver(r) // WAI when config.Saver==nil
+}