Package Format

Goal: define the format for a package

* Should work with the [content-digest](https://github.com/bytecodealliance/SIG-Registries/blob/main/proposals/Publish-API.md#content-digest) field
* multi-file bundling or how to associate recognized package types with each other.
* Should this include component validation? For example if the package type is a Component, then it should adhere to [binary spec](https://github.com/WebAssembly/component-model/blob/main/design/mvp/Binary.md) of the Component Model. The same is true for a core module (like the cpython interpreter).

Note: Signature formats do not need to be defined as a part of this (covered by #26).

Read the linked Publish-API.md content-digest section and the WebAssembly Component Model Binary.md specification first. Define the package format's digest relationship, multi-file bundling and recognized package types, including the scope of component and core-module validation. Done means these decisions are captured in an agreed specification; signature formats remain out of scope per #26.

Written by the indexing model from the issue text.