gopls: v0.7.5: add replace directive back to go.mod

In preparation for merging master, add back the x/tools replace
directive.

For golang/go#50613

Change-Id: Icb06d15e256396d3e030aec644841f6acc1edcea
Reviewed-on: https://go-review.googlesource.com/c/tools/+/378582
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Suzy Mueller <suzmue@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/go.mod

@@ -22,3 +22,5 @@ require (
 	golang.org/x/text v0.3.7 // indirect
 	golang.org/x/xerrors v0.0.0-20200804184101-5ec99f83aff1 // indirect
 )
+
+replace golang.org/x/tools => ../