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/iox/iox.go

@@ -1,58 +0,0 @@
-// Package iox contains io extensions.
-package iox
-
-import (
-	"context"
-	"io"
-)
-
-// ReadAllContext is like io.ReadAll but reads r in a
-// background goroutine. This function will return
-// earlier if the context is cancelled. In which case
-// we will continue reading from r in the background
-// goroutine, and we will discard the result. To stop
-// the long-running goroutine, you need to close the
-// connection bound to the r reader, if possible.
-func ReadAllContext(ctx context.Context, r io.Reader) ([]byte, error) {
-	datach, errch := make(chan []byte, 1), make(chan error, 1) // buffers
-	go func() {
-		data, err := io.ReadAll(r)
-		if err != nil {
-			errch <- err
-			return
-		}
-		datach <- data
-	}()
-	select {
-	case data := <-datach:
-		return data, nil
-	case <-ctx.Done():
-		return nil, ctx.Err()
-	case err := <-errch:
-		return nil, err
-	}
-}
-
-// CopyContext is like io.Copy but may terminate earlier
-// when the context expires. This function has the same
-// caveats of ReadAllContext regarding the temporary leaking
-// of the background goroutine used to do I/O.
-func CopyContext(ctx context.Context, dst io.Writer, src io.Reader) (int64, error) {
-	countch, errch := make(chan int64, 1), make(chan error, 1) // buffers
-	go func() {
-		count, err := io.Copy(dst, src)
-		if err != nil {
-			errch <- err
-			return
-		}
-		countch <- count
-	}()
-	select {
-	case count := <-countch:
-		return count, nil
-	case <-ctx.Done():
-		return 0, ctx.Err()
-	case err := <-errch:
-		return 0, err
-	}
-}