[gopls-release-branch.0.6] add replace to go.mod

To run tests.

Change-Id: I42cbc803cb1144e9aca68c76603561cc40f96f07
Reviewed-on: https://go-review.googlesource.com/c/tools/+/292495
Trust: Heschi Kreinick <heschi@google.com>
Run-TryBot: Heschi Kreinick <heschi@google.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
diff --git a/gopls/go.mod b/gopls/go.mod
index 00fb00c..a82da63 100644
--- a/gopls/go.mod
+++ b/gopls/go.mod
@@ -14,3 +14,5 @@
 	mvdan.cc/gofumpt v0.1.0
 	mvdan.cc/xurls/v2 v2.2.0
 )
+
+replace golang.org/x/tools => ../