Use index section as resources

As Bundle Dependencies proposal briefly mentions, it would be nice that a resource listed in the index section of the bundle will be also a target of subresource loading, even if the resource is not mentioned in resources in <script type=webbundle>.

The benefit is: users don't have to list all resources in resources. They have to specify only direct dependencies.

For example, suppose that main.html is:

<script type="webbundle">
{
   source: "a.wbn",
   resources: [....]    # what should be specified?
}
</script>

<link rel="style" href="a.css" />

...

<img src="a.png" />

...

<script type="module" src="a1.js" />

The bundle, a.wbn has the following resources:

The dependency graph is:

main.html
├── a1.js
│   ├── a2.js
│   └── a3.js
├── a.png
└── a.css

Then, it would be nice that users can specify only the direct dependencies in resources, as follows:

<script type="webbundle">
{
   source: "a.wbn",
   resources: ["a1.js", "a.css", "a.png"]
}
</script>

<link rel="style" href="a.css" />

...

<img src="a.png" />

...

<script type="module" src="a1.js" />

Instead of listing all resources:

<script type="webbundle">
{
   source: "a.wbn",
   resources: ["a1.js", "a2.js", "a3.js", "a.css", "a.png"]
}
</script>

<link rel="style" href="a.css" />

...

<img src="a.png" />

...

<script type="module" src="a1.js" />

We probably want to introduce an optional key in JSON of <script type=webbundle> to enable this behavior of not, however, I'm feeling that this behavior is fine as the default behavior in most cases .

Start with the linked Bundle Dependencies proposal and the webbundle script examples in this issue. Decide whether index entries should serve as subresource-loading targets by default or behind an optional JSON key, then document the agreed behavior in the relevant Web Bundle proposal or specification. Done means the dependency graph and resources/index semantics are unambiguous.