go/packages/testdata: add go directives to fake module files

Stop fighting the behavior of the go tool when run in these directories.

Updates golang/go#30790

Change-Id: I32dfeb0bafa3ed3664500f1768b2293e5257d09b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/168757
Run-TryBot: Michael Matloob <matloob@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

Author: matloob

go/packages/testdata/TestName_Modules/pkg/mod/github.com/heschik/tools-testrepo/v2@v2.0.0/go.mod

@@ -1 +1,3 @@
 module github.com/heschik/tools-testrepo/v2
+
+go 1.12

go/packages/testdata/TestName_Modules/pkg/mod/github.com/heschik/tools-testrepo@v1.0.0/go.mod

@@ -1 +1,3 @@
 module github.com/heschik/tools-testrepo
+
+go 1.12

go/packages/testdata/TestName_ModulesDedup/pkg/mod/github.com/heschik/tools-testrepo/v2@v2.0.1/go.mod

@@ -1 +1,3 @@
 module github.com/heschik/tools-testrepo/v2
+
+go 1.12

go/packages/testdata/TestName_ModulesDedup/pkg/mod/github.com/heschik/tools-testrepo/v2@v2.0.2/go.mod

@@ -1 +1,3 @@
 module github.com/heschik/tools-testrepo/v2
+
+go 1.12

go/packages/testdata/TestName_ModulesDedup/pkg/mod/github.com/heschik/tools-testrepo@v1.0.0/go.mod

@@ -1 +1,3 @@
 module github.com/heschik/tools-testrepo
+
+go 1.12