allow delay_load list to list multiple environments

[t-kalinowski]

This is an internal change allowing usage like this in tensorflow, where length(environment) > 1:

tf <- NULL
.onLoad <- function(libname, pkgname) {
   ...
   tf <<- import("tensorflow", delay_load = list(
     priority = 5, # keras sets priority = 10
     environment = c("r-tensorflow", "r-keras"),
     ...

No doc changes because this mechanism for expressing soft import preferences is overly complex because of legacy support for both conda and virtual envs, (and the priority mechanism is not exposed intentionally).

Most package authors looking to accomplish something similar would be better served by multiple calls to use_python(,required = FALSE), e.g.

tf <- NULL
.onLoad <- function(libname, pkgname) {
   ...
   use_virtualenv("r-tensorflow", required = FALSE)
   use_virtualenv("r-keras", required = FALSE)
   use_condaenv("r-tensorflow", required = FALSE)
   use_condaenv("r-keras", required = FALSE)

   tf <<- import("tensorflow", delay_load = TRUE)
   ...