Thread files from image's ancestors all the way through to it's output

[bcmyers]

As you construct images by layering them on top of one another, e.g.

oci_image(
   name = "image1",
   base = "@ubuntu//image",
   layers = [":layer1"],
)

oci_image(
  name = "image2",
  base = ":image1",
  layers = [":layer2"],
)

You want the images to keep passing the files of their ancestors along the chain.

This is not expensive (because we are using depset) and it will enable future improvements, in particular writing an oci_image_tar rule that knows how to turn an oci_image target into a tar file that conforms to the OCI Image Layout spec and can be loaded into local Docker. This PR sets the groundwork for that kind of functionality since, with this change, image2's output in the above example will now contain all the files from @base//image, layer1 and layer2, which are all required to create a tarball. Before this change, image2's output only contains the files from layer2

[joneshf-dd]

Looks good for Language Tools!

👏 praise: Thanks for fixing this! Very excited about what this unlocks!