No option to preserve symlinks in pkg_tar

The current behavior of `pkg_tar` is that if I have a file and a symlink to that file, and both are given as sources, the symlink is "expanded" and two copies of the original file are added to the repo.

For some context, my use case is that I'm trying to package an external toolchain's libc into a tar for use in Docker containers (cross-compiling is... fun). As you can imagine, this is massively ballooning the size of my images.

While I could certainly attempt to do something like filtering all the links out and then generating a `symlinks` dict to pass in, I was wondering if there would be interest in adding a flag to allow this behavior in the main rule. I think it could be useful in a variety of contexts, especially in generating library debs for use by others.

The behavior as I imagine it would be along the lines of passing a `preserve_symlinks` flag, which would then include symlinks *if* they only point to files in the archive. This validation should be simple enough, and would remove risks of non-hermeticity. Alternatively, symlinks pointing elsewhere could follow the current behavior, while hermetic symlinks could be preserved.

Please let me know if there are any concerns I'm missing, or if there's an obvious solution to my problem that I missed somewhere in the docs.