cmd/go: add a source file in the multiple-paths module in TestScript/mod_tidy_replace

This situation is analogous to CL 309334: the test expects 'go mod
tidy' to fail due to a module used for more than one path in the build
list, but doesn't actually contain any packages or imports — so no
module is necessarily used at all, and the error only occurs if we
report it prematurely.

For #36460

Change-Id: I5ccecf30f280895eba913a8d62571872b75e710d
Reviewed-on: https://go-review.googlesource.com/c/go/+/312098
Trust: Bryan C. Mills <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>

Author: Bryan C. Mills

src/cmd/go/testdata/script/mod_tidy_replace.txt

@@ -136,3 +136,10 @@ require (
 )
 
 replace not-rsc.io/quote/v3 => rsc.io/quote/v3 v3.0.0
+-- multiple-paths/use.go --
+package quoter
+
+import (
+	_ "not-rsc.io/quote/v3"
+	_ "rsc.io/quote/v3"
+)