Automatic R package download does not work with custom repositories

[georgestagg]

See discussion in r-wasm/actions#18 for details.

The issue is that at app startup renv searches for packages inside the Shinylive app. Then, we then download each of them from the default webR binary repository. However, we might not want to download all packages from the default repo -- we might want to load them from a personal repo built with r-wasm/actions, R Universe, or somewhere else entirely.

The relevant R code:

shinylive/src/hooks/useWebR.tsx

Lines 256 to 267 in 6afcdc8

	# Uniquely install packages with webr
	unique_pkgs <- unique(renv::dependencies(appDir, quiet = TRUE)$Package)
	lapply(unique_pkgs, function(pkg_name) {
	if (isTRUE(.webr_pkg_cache[[pkg_name]])) return()
	has_pkg <- nzchar(system.file(package = pkg_name))
	.webr_pkg_cache[[pkg_name]] <<- has_pkg
	if (!has_pkg) {
	webr::install(pkg_name)
	}
	})

We should have some way to signal to Shinylive that we have a package from a custom repository we want to load. Some immediate options come to mind, in order of my personal preference.

1. Setup a new user library at startup so that webr::install() has priority over packages that are downloaded automatically.
2. Somehow allow for defining a custom repository list for the automatic package download system.
3. Document that automatic packages can be removed with webr::unmount() (P.S. Should we shim remove.packages() to do this in any case?)

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Note: This issue should not affect Python apps. R apps deployed with shinylive::export() will be unaffected going forward, since in a future releases of r-shinylive the shinylive::export() function will gather the package references for packages installed on the local machine and deploy them directly with the app.