Normalize the paths of all files, avoiding some duplicate misses of directories.

For example with Products.PloneHelpCenter (on github) I get this
report, where in all cases the directories are there and have a file
in them:

missing from VCS:
  Products/PloneHelpCenter/profiles/default/workflows/helpcenter_workflow/
  Products/PloneHelpCenter/profiles/default/workflows/helpcenterfolder_workflow/
  Products/PloneHelpCenter/profiles/default/workflows/helpcentersubitem_workflow/
missing from sdist:
  Products/PloneHelpCenter/profiles/default/workflows/helpcenter_workflow
  Products/PloneHelpCenter/profiles/default/workflows/helpcenterfolder_workflow
  Products/PloneHelpCenter/profiles/default/workflows/helpcentersubitem_workflow
Replace strip_slashes with normalize_names in test.
Merge pull request #16 from mauritsvanrees/maurits-fix-missing-directories

Normalize the paths of all files, avoiding duplicate misses of directories