FR: Syntax sugar `select` generated by oci_pull is a common pitfall.

I have seen countless users falling into the syntax sugar `select` trap generated by oci_pull. They either miss to add a transition or expect `architecture` and `os` to on the `oci_image` do the transition automatically for them.

I have no idea how widespread the optional toolchains feature of bazel is, therefore can't say it's time to implement transitions in `oci_image_index`. i don't want users to see bunch of toolchain issues which they can't make sense of.

There should be a warning to users about the select statement doing the wrong thing if there's no transitions.

Start by tracing the select syntax sugar generated by oci_pull and how oci_image consumes architecture and os, then compare the role of transitions in oci_image_index and optional toolchains. The work is done when users receive a clear warning when select is used without the required transitions; the issue does not name a file or test to run.

Written by the indexing model from the issue text.