cmd/cover: add go.mod file in lineDupDir

This fixes TestFuncWithDuplicateLines (introduced in CL 167257)
in module mode.

Updates #30746
Updates #30228

Change-Id: I7b3e7192ae23f855c373e881389874ff6ffd49ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/167740


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>