package copy fails if symlinks in root dir

[nerdvegas]

Happens in non-varianted packages only:

  File "/sw/dev/managani/packages/rez/2.26.0/python-2/python/rez/package_copy.py", line 189, in copy_package
    overrides=overrides
  File "/sw/dev/managani/packages/rez/2.26.0/python-2/python/rez/package_copy.py", line 270, in _copy_variant_payload
    replacing_copy(src_path, dest_path)
  File "/sw/dev/managani/packages/rez/2.26.0/python-2/python/rez/utils/filesystem.py", line 138, in replacing_copy
    shutil.copy2(src, tmp_dest)
  File "/sw/packages/external/python/2.7.13/platform-linux/arch-x86_64/os-CentOS-7/lib/python2.7/shutil.py", line 130, in copy2
    copyfile(src, dst)
  File "/sw/packages/external/python/2.7.13/platform-linux/arch-x86_64/os-CentOS-7/lib/python2.7/shutil.py", line 82, in copyfile
    with open(src, 'rb') as fsrc:
IOError: [Errno 21] Is a directory: '/sw/packages/internal/dummy/2.4.24/ext'

[nerdvegas]

#552