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/operations.go

@@ -0,0 +1,45 @@
+package netxlite
+
+// Operations that we measure. They are the possibly values of
+// the ErrWrapper.Operation field.
+const (
+	// ResolveOperation is the operation where we resolve a domain name.
+	ResolveOperation = "resolve"
+
+	// ConnectOperation is the operation where we do a TCP connect.
+	ConnectOperation = "connect"
+
+	// TLSHandshakeOperation is the TLS handshake.
+	TLSHandshakeOperation = "tls_handshake"
+
+	// QUICHandshakeOperation is the handshake to setup a QUIC connection.
+	QUICHandshakeOperation = "quic_handshake"
+
+	// QUICListenOperation is when we open a listening UDP conn for QUIC.
+	QUICListenOperation = "quic_listen"
+
+	// HTTPRoundTripOperation is the HTTP round trip.
+	HTTPRoundTripOperation = "http_round_trip"
+
+	// CloseOperation is when we close a socket.
+	CloseOperation = "close"
+
+	// ReadOperation is when we read from a socket.
+	ReadOperation = "read"
+
+	// WriteOperation is when we write to a socket.
+	WriteOperation = "write"
+
+	// ReadFromOperation is when we read from an UDP socket.
+	ReadFromOperation = "read_from"
+
+	// WriteToOperation is when we write to an UDP socket.
+	WriteToOperation = "write_to"
+
+	// UnknownOperation is when we cannot determine the operation.
+	UnknownOperation = "unknown"
+
+	// TopLevelOperation is used when the failure happens at top level. This
+	// happens for example with urlgetter with a cancelled context.
+	TopLevelOperation = "top_level"
+)