Logging calls when running "go install -a std" turns:

  547  openDir succeeded
  3593 openDir failed and fell back to openFile
  3592 openFile succeeded
  1    both failed

into:

  3592 openFile succeeded
  548  openFile failed and fell back
  547  openDir succeeded
  1    both failed

Here the change trades 3593 failed openDir for 548 failed openFile.

Fix issue 7426.

LGTM=alex.brainman
R=golang-codereviews, alex.brainman, bradfitz
CC=golang-codereviews
https://golang.org/cl/70480044