[TarFile]: normalize_path does not normalize when --directory is present

[grrtrr]

Problem description

When using --directory (self.directory) with TarFile, the following segment produces non-normalized path components:

# pkg/private/tar/build_tar.py
    # This prevents a potential problem for users with both a prefix_dir and
    # symlinks that also repeat the prefix_dir. The old behavior was that we
    # would get just the symlink path. Now we are prefixing with the prefix,
    # so you get the file in the wrong place.
    # We silently de-dup that. If people come up with a real use case for
    # the /a/b/a/b/rest... output we can start an issue and come up with a
    # solution at that time.
    if self.directory and not dest.startswith(self.directory):
      dest = self.directory + dest

We are using --directory, and so I was seeing paths like this:

runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/__init__.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/_cmp.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/_typing_compat.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/_version_info.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/converters.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/exceptions.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/filters.pyi
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/py.typed
runtime/_main/../_main~ps~av_pip_attrs_3_10/pypi__attrs/attr/setters.pyi
...

Note that these collapse into runtime/_main~ps~av_pip_attrs_3_10/pypi__attrs/....

This may normally not be a problem, but becomes an issue when piping the tar file into mksquashfs:

Parallel mksquashfs: Using 32 processors
Creating 4.0 filesystem on bazel-out/k8-fastbuild/<redacted>.squashfs, block size 131072.
[=======================|                                                                                                                                                                       ]  1911/15662  12%
FATAL ERROR: Error: Tar pathname can't have '.' or '..' in it

What to do

Call the existing normpath() on the concatenated path.