Support importmaps inside index.html

**Is your feature request related to a problem? Please describe.**
I want to bundle a browser ready website which has an importmap defined as part of its index.html. Currently browsers do not support externally defined importmaps, but they most be inlined into the .html file.

It would be really cool, if one could package such a self-described website.

**Describe the solution you'd like**
While searching through the web, it seams that packup with its deno support and `--import-map` flag for external importmaps is closest to fulfilling this role of being able to pack a website .html with importmaps defined inside.

It would be great if packup could support this.

**Describe alternatives you've considered** I am considering writing a custom resolver for parcel, but that is quite tough. I haven't found any ready solution for this. jspm.io has other problems and "bundles" importmaps by not bundling them, which is not an option for me. I really like to bundle all dependencies in order to not rely on other cdn being available for production use.

No source file or test is identified. Start by locating the HTML packaging entry point and the existing --import-map handling, then define a reproducible index.html importmap case and tests that confirm its dependencies are bundled.

Written by the indexing model from the issue text.