This will allow to align the loader package's implementations with the
loader-core's regarding retry policy (no retry policy on the package ones).
Also, as a next step, this will allow again to simplify the loader-core's
This should help in fixing or at least decreasing the frequency of  (which happens for all package loaders, not only cran's).
The diff is a tad big because of tests.
Related to D2511 (which only added content_add)