[gopls-release-branch.0.7] gopls/go.mod: add back the replace directive while staging cherry-picks

Change-Id: I0e671c3d7d8b9df2737ba39a6684e74fc19dcded
Reviewed-on: https://go-review.googlesource.com/c/tools/+/348373
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
diff --git a/gopls/go.mod b/gopls/go.mod
index 7374404..487499d 100644
--- a/gopls/go.mod
+++ b/gopls/go.mod
@@ -19,3 +19,5 @@
 	mvdan.cc/gofumpt v0.1.1
 	mvdan.cc/xurls/v2 v2.3.0
 )
+
+replace golang.org/x/tools => ../