Support for online `htmlDependency()`

Discussed in https://github.com/quarto-dev/quarto-cli/discussions/10325

^{Originally posted by walkerke July 18, 2024}

Description

I'm developing a package called mapgl, installable at remotes::install_github("walkerke/mapgl"). For licensing reasons, I'm bringing in the Mapbox GL JS JavaScript and CSS assets in via CDN rather than bundling them within the package. However, this gives Quarto problems:

---
title: "quarto_test"
format: html
editor: visual
---

```{r}
library(mapgl)

mapboxgl()
```

Gives:

Error in FUN(X[[i]], ...) : 
  Dependency mapbox-gl-js 3.4.0 is not disk-based
Calls: .main ... dependencies_from_render -> html_dependencies_as_string -> lapply -> FUN
Execution halted

I know this isn't a problem with my package per se, as disk-based assets render fine in Quarto. However - I do notice that R Markdown does not have the same problem (see my package documentation here, which I built in R Markdown for that reason).

Is there any workaround you know of to be able to pull in external assets via CDN? Or, does this merit filing an issue?