cmd/go/internal/modload: use lockedfile to read path-replacement go.mod files

When parsing go.mod files found via file-path replacements, it's safer to
use lockedfile.Read instead of ioutil.ReadFile, in case of overwriting by
other concurrent go commands.

Change-Id: I7dcac3bb5ada84bee1eb634b39f813c461ef103a
Reviewed-on: https://go-review.googlesource.com/c/go/+/230838
Reviewed-by: Bryan C. Mills <[email protected]>
Reviewed-by: Jay Conrod <[email protected]>
Run-TryBot: Bryan C. Mills <[email protected]>

Author: Richard Miller

Diff for: src/cmd/go/internal/modload/mvs.go

@@ -7,14 +7,14 @@ package modload
 import (
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"os"
 	"path/filepath"
 	"sort"
 	"sync"
 
 	"cmd/go/internal/base"
 	"cmd/go/internal/cfg"
+	"cmd/go/internal/lockedfile"
 	"cmd/go/internal/modfetch"
 	"cmd/go/internal/mvs"
 	"cmd/go/internal/par"
@@ -108,7 +108,7 @@ func (r *mvsReqs) required(mod module.Version) ([]module.Version, error) {
 				dir = filepath.Join(ModRoot(), dir)
 			}
 			gomod := filepath.Join(dir, "go.mod")
-			data, err := ioutil.ReadFile(gomod)
+			data, err := lockedfile.Read(gomod)
 			if err != nil {
 				return nil, fmt.Errorf("parsing %s: %v", base.ShortPath(gomod), err)
 			}