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/cmd/jafar/badproxy/badproxy.go

@@ -16,7 +16,7 @@ import (
 	"time"
 
 	"github.com/google/martian/v3/mitm"
-	"github.com/ooni/probe-cli/v3/internal/netxlite/iox"
+	"github.com/ooni/probe-cli/v3/internal/netxlite"
 )
 
 // CensoringProxy is a proxy that does not behave correctly.
@@ -57,7 +57,7 @@ func (p *CensoringProxy) serve(conn net.Conn) {
 	} else {
 		const maxread = 1 << 17
 		reader := io.LimitReader(conn, maxread)
-		iox.ReadAllContext(context.Background(), reader)
+		netxlite.ReadAllContext(context.Background(), reader)
 	}
 	conn.Close()
 }