Get the modules aware importer via the function that was available in Go 1.11 and earlier

.travis.gogenerate.sh

@@ -4,6 +4,7 @@
 # automatically or via the GO111MODULE=on environment variable. Codegen only
 # works with modules, so skip generation if modules is not in use.
 if [[ -z "$(go env GOMOD)" ]]; then
+  echo "Skipping go generate because modules not enabled and required"
   exit 0
 fi
 

_codegen/go.mod

@@ -1,5 +1,5 @@
 module github.com/stretchr/testify/_codegen
 
-go 1.13
+go 1.11
 
 require github.com/ernesto-jimenez/gogen v0.0.0-20180125220232-d7d4131e6607

_codegen/main.go

@@ -194,7 +194,7 @@ func parsePackageSource(pkg string) (*types.Scope, *doc.Package, error) {
 	}
 
 	cfg := types.Config{
-		Importer: importer.ForCompiler(fset, "source", nil),
+		Importer: importer.For("source", nil),
 	}
 	info := types.Info{
 		Defs: make(map[*ast.Ident]types.Object),