go.tools/go/loader: add file missing from rev ebef51b726fc

(CL https://golang.org/cl/62540043)

TBR=gri
R=gri
CC=golang-codereviews
https://golang.org/cl/65530046

go/loader/backdoor.go

@@ -0,0 +1,13 @@
+package loader
+
+import (
+	"go/ast"
+	"go/build"
+	"go/token"
+)
+
+// PackageLocatorFunc exposes the address of parsePackageFiles to tests.
+// This is a temporary hack until we expose a proper PackageLocator interface.
+func PackageLocatorFunc() *func(ctxt *build.Context, fset *token.FileSet, path string, which string) ([]*ast.File, error) {
+	return &parsePackageFiles
+}