Changes to the cache API required by the new reticulate cache implementation

[leogama]

As we discussed in rstudio/reticulate#1210 (comment), these changes use the same interface for the cache of external engines as the R cache interface. Relevant changes for non-R chunks:

• cache_engines stores cache importer functions that, upon call, return "cache objects", i.e. lists with functions as items (exists, load, save, purge, etc.)
• external cache functions are called in the knit() call's working directory
• cache checks (exists()) and purges are done for R cache and external cache together for consistency

Plus a bug fix: always check for .RData files (which stores the output in lazy cache mode if I understood it right).

---

@yihui You may review this PR right away, but let's hold the merge while I'm still implementing and testing the reticulate PR.

[CLAassistant]

All committers have signed the CLA.

[leogama]

The last commit should work with rstudio/reticulate@5d6f7a7 and uqfoundation/dill@71b4f77

[leogama]

Updated: The last commit should work with rstudio/reticulate@a33ed39 and uqfoundation/dill@4244d3b

[yihui]

Looks good over all. I'll make a few cosmetic changes and merge later. Thank you!

[yihui]

Note that glob_path could be a vector of length > 1, so we still need to purge them one by one via a loop.

[yihui]

Oh actually since the API has changed, we need to update the documentation accordingly:

knitr/R/engine.R

Lines 35 to 51 in 6bfffe9

	#' Cache engines of other languages
	#'
	#' This object controls how to load cached environments from languages other
	#' than R (when the chunk option \code{engine} is not \code{'R'}). Each
	#' component in this object is a function that takes the current path to the
	#' chunk cache and loads it into the language environment.
	#'
	#' The cache engine function has one argument \code{options}, a list containing
	#' all chunk options. Note that \code{options$hash} is the path to the current
	#' chunk cache with the chunk's hash, but without any file extension, and the
	#' language engine may write a cache database to this path (with an extension).
	#'
	#' The cache engine function should load the cache environment and should know
	#' the extension appropriate for the language.
	#' @references See \url{https://github.com/rstudio/reticulate/pull/167} for an
	#' implementation of a cache engine for Python.
	#' @export

Could you please also do that? Thank you!