Detect integrations injected by another theme

In a recent update, there was a utility added to query what integrations are inside the project:

```ts
import { integrations } from 'my-theme:context'

if (integrations.has('@inox-tools/sitemap-ext')) {
	// ...
}
```

But, this utility cannot detect integrations added by another theme. This utility should also be able to detect integrations added by other themes.