load dependencies of external libraries from external lib

Hello,

When the option load.external.packages.on.startup is TRUE, packrat loads the dependent libraries from the private lib folder if they are there. I think it would be better to load them from the external lib as well.

It seems that they are loaded here:

 if (isTRUE(opts$load.external.packages.on.startup())) {
    lapply(opts$external.packages(), function(x) {
      library(x, character.only = TRUE, quietly = TRUE)
    })
  }

Then my suggestion is to replace this code with:

 if (isTRUE(opts$load.external.packages.on.startup())) {
    lapply(opts$external.packages(), function(x) {
      library(x, character.only = TRUE, quietly = TRUE, lib.loc=packrat:::libExtDir())
    })
  }

Or to use packrat::extlib.

It seems that there's the same "problem" with packrat::with_extlib.

Start in R/packrat-mode.R at the startup loading block linked in the issue, then inspect packrat::extlib and packrat::with_extlib to understand how external libraries are selected. Verify that dependencies of external packages load from the external library when load.external.packages.on.startup is TRUE, including the related with_extlib behavior.