HTMLManager uses hardcoded entrypoint (dist/<name>.js) for extensions

[timkpaine]

Description

HTMLManager is hardcoded to pull from dist/<package name>

We should inspect the main attribute of the package.json if possible to identify the entrypoint.

Reproduce

use e.g. perspective which emits to dist/esm/

This likely won't work anyway due to use of esm, but just an example.

Expected behavior

Use the extension's preferred entry point, at the bare minimum main but ideally per-cdn.

Context

#2381
voila-dashboards/voila#1128
voila-dashboards/voila#1154
bqplot/bqplot#1495