[lldb/Utility] Don't forward directories to the file collector

The VFS mapping writer assumes that all the paths it gets are files.
When passed a directory, it ends up as a file in the VFS mapping twice,
once as a file and once as a directory.

  {
    'type': 'file',
    'name': "Output",
    'external-contents': "/root/path/to/Output"
  },
  {
    'type': 'directory',
    'name': "Output",
    'contents': [ ... ]
  }