multi: break invoice depenency on channeldb

Now that we have the new package `lnd/channeldb/models` we can invert the
depenency between `channeldb` and `invoices`.

- Move all the invoice related types and errors to the
`invoices` package.

- Ensure that all the packages dealing with invoices use the types and
  interfaces defined in the `invoices` package.

- Implement the InvoiceDB interface (defined in `lnd/invoices`) in
  channeldb.

- Add new mock for InterfaceDB.

- `InvoiceRegistery` tests are now in its own subpacakge (they need to
  import both invoices & channeldb). This is temporary until we can
  decouple them.

rpcserver.go

@@ -5507,7 +5507,7 @@ func (r *rpcServer) LookupInvoice(ctx context.Context,
 
 	invoice, err := r.server.invoices.LookupInvoice(payHash)
 	switch {
-	case err == channeldb.ErrInvoiceNotFound:
+	case errors.Is(err, invoices.ErrInvoiceNotFound):
 		return nil, status.Error(codes.NotFound, err.Error())
 	case err != nil:
 		return nil, err
@@ -5551,7 +5551,7 @@ func (r *rpcServer) ListInvoices(ctx context.Context,
 
 	// Next, we'll map the proto request into a format that is understood by
 	// the database.
-	q := channeldb.InvoiceQuery{
+	q := invoices.InvoiceQuery{
 		IndexOffset:    req.IndexOffset,
 		NumMaxInvoices: req.NumMaxInvoices,
 		PendingOnly:    req.PendingOnly,