FR: implement flatten_tars macro

[thesayyn]

Needed to flatten multiple layers into one;

Features

• takes multiple tar files and spits out a single flattened layer

[thesayyn]

this turned out to be super simple with pkg_tar.

pkg_tar(
  name = "flatten",
  deps = ["layer1.tar", "layer2.tar"]
)

[thesayyn]

closing as theres an obvious alternative.

[Place1]

I've run into this question as well but found I needed to use deps instead of srcs like this:

load("@rules_pkg//pkg:tar.bzl", "pkg_tar")

pkg_tar(
  name = "flatten",
  deps = ["layer1.tar", "layer2.tar"]
)