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 <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>