fix: Fall back to directory based runfiles using relative paths

[mering]

This will also work out of the box when the runfiles tree is packaged into a container image as follows:

py_binary(
    name = "binary",
    srcs = ["main.py"],
    data = ["runfile.txt"],
)

pkg_tar(
    name = "layer",
    srcs = [":binary"],
    strip_prefix = "/",
    include_runfiles = True,
)

oci_image(
    name = "image",
    tars = [":layer"],
)

[rickeylev]

Thanks for the fix @mering . I think this looks fine, but some surrounding changes to make:

• Can you describe the problematic case and why this fixes it?
• Tests: This looks easy to test if we just have a shell script setup the environment (unset the RUNFILES env vars) and call the binary and check it worked. The tests/support/sh_py_run_test.bzl#sh_py_run_test should make this pretty easy. There are examples of using it in tests/bootstrap_impls
• CHANGELOG.md needs to be updated.

[mering]

@rickeylev Thanks for the review. See my answers below:

• When used in a container image without involving Bazel at all, the environment variables are not set but runfiles are in the expected folder BIN.runfiles as it was packaged via pkg_tar with include_runfiles = True. I updated the description with an example.
• I tried doing this but for some reason the $RUNFILES_DIR is still set within Python even though I unset it in the shell script.
• I added this to the CHANGELOG.