internal/lsp: support go1.12

We still don't have a good way to make sure that we don't break 1.12
support, but this is an easy enough fix to start with.

Updates golang/go#39146

Change-Id: I14fe997fa1f3d60320d77e664208e25d97ae6f4f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/243578
Run-TryBot: Rebecca Stambler <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: stamblerre

internal/jsonrpc2/messages.go

@@ -6,8 +6,9 @@ package jsonrpc2
 
 import (
 	"encoding/json"
-	"errors"
 	"fmt"
+
+	errors "golang.org/x/xerrors"
 )
 
 // Message is the interface to all jsonrpc2 message types.

internal/jsonrpc2/serve.go

@@ -6,14 +6,14 @@ package jsonrpc2
 
 import (
 	"context"
-	"errors"
 	"fmt"
 	"io"
 	"net"
 	"os"
 	"time"
 
 	"golang.org/x/tools/internal/event"
+	errors "golang.org/x/xerrors"
 )
 
 // NOTE: This file provides an experimental API for serving multiple remote