imports: stub out dirPackageInfo function in tests

TestFixImports did not have to explicitly add "strings" to the
simplePkgs map - this only happened because sibling files were searched
and the strings import was found through them. If we stub out the
dirPackageInfo function, we have to explicitly add the strings import.

Change-Id: I149e522fad37039fc790e6a855834d86ff050674
Reviewed-on: https://go-review.googlesource.com/129036
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

imports/fix_test.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"context"
 	"flag"
+	"fmt"
 	"go/build"
 	"io/ioutil"
 	"os"
@@ -1066,16 +1067,22 @@ func TestFixImports(t *testing.T) {
 		"regexp":    "regexp",
 		"snappy":    "code.google.com/p/snappy-go/snappy",
 		"str":       "strings",
+		"strings":   "strings",
 		"user":      "appengine/user",
 		"zip":       "archive/zip",
 	}
-	old := findImport
+	oldFindImport := findImport
+	oldDirPackageInfo := dirPackageInfo
 	defer func() {
-		findImport = old
+		findImport = oldFindImport
+		dirPackageInfo = oldDirPackageInfo
 	}()
 	findImport = func(_ context.Context, pkgName string, symbols map[string]bool, filename string) (string, bool, error) {
 		return simplePkgs[pkgName], pkgName == "str", nil
 	}
+	dirPackageInfo = func(_, _, _ string) (*packageInfo, error) {
+		return nil, fmt.Errorf("no directory package info in tests")
+	}
 
 	options := &Options{
 		TabWidth:  8,