feat(netxlite): use *Netx for NewParallelUDPResolver (#1257)

The overall idea at this point is that we will have an interface
containing the methods we currently have inside *Netx and we will sever
the hard dependency between measurexlite and netxlite.

We will probably still need to import some contants from netxlite but,
for all effects and purposes, measurexlite would become completely
mockable (as it is now) with an explicit dependency on the related
behavior.

In turn, this refactoring would clarify which netxlite functionality we
use for measuring, which are legacy, and which we use for communicating
with the OONI backend. That change would definitely make it obvious to
modify netxlite and go in the overall direction defined by
ooni/probe#2531.

internal/netxlite/resolvercore.go

@@ -94,12 +94,19 @@ func NewSerialUDPResolver(logger model.DebugLogger, dialer model.Dialer, address
 // - dialer is the dialer to create and connect UDP conns
 //
 // - address is the server address (e.g., 1.1.1.1:53)
-func NewParallelUDPResolver(logger model.DebugLogger, dialer model.Dialer, address string) model.Resolver {
+func (netx *Netx) NewParallelUDPResolver(logger model.DebugLogger, dialer model.Dialer, address string) model.Resolver {
 	return WrapResolver(logger, NewUnwrappedParallelResolver(
 		wrapDNSTransport(NewUnwrappedDNSOverUDPTransport(dialer, address)),
 	))
 }
 
+// NewParallelUDPResolver is equivalent to creating an empty [*Netx]
+// and calling its NewParallelUDPResolver method.
+func NewParallelUDPResolver(logger model.DebugLogger, dialer model.Dialer, address string) model.Resolver {
+	netx := &Netx{Underlying: nil}
+	return netx.NewParallelUDPResolver(logger, dialer, address)
+}
+
 // WrapResolver creates a new resolver that wraps an
 // existing resolver to add these properties:
 //