Add message before PACKAGES is constructed after removing packages

[JosiahParry]

It could allay concerns to provide an informative message that the PACKAGES file needs to be rewritten after packages have been removed.

Related #29

[georgestagg]

There is already a similar message printed by tools::write_PACKAGES() whenever the PACKAGES file is reconstructed. The function is called from here:

rwasm/R/repo.R

Lines 280 to 284 in 32a757e

	update_packages <- function(contrib_src, contrib_bin) {
	tools::write_PACKAGES(contrib_src, verbose = TRUE)
	tools::write_PACKAGES(contrib_bin, verbose = TRUE, type = "mac.binary")
	invisible(NULL)
	}

and the output looks something like the following, beginning with "Processing packages".

Processing packages:
  /private/tmp/nloptr/repo/bin/emscripten/contrib/4.3/lme4_1.1-35.2.tgz
  /private/tmp/nloptr/repo/bin/emscripten/contrib/4.3/nloptr_2.0.3.tgz
done

So, I think if we wanted to do this we'd need a few changes:

1. Print our custom message in update_packages(), just before calling tools::write_PACKAGES().

2. Expand our message to include more than "Processing packages" or "Reconstructing PACKAGES." We should explain what this actually means.

3. Perhaps pass the verbose argument into the function (defaulting to TRUE, the output is generally useful) rather than just hard coding verbose = TRUE, so that we can easily squelch both our custom message and the verbose output from tools::write_PACKAGES().