[Rd] Improving messaging on successful binary package installation

[Jeroen Ooms]

I have been testing R-devel and really enjoying fast parallel
downloading of R packages, especially on mac/win where there is no
complication step either, it is super nice.

Perhaps the install.packages() messaging could still be improved a
bit. Right now if one installs a package on Windows/MacOS we see:

  > install.packages('V8')
  also installing the dependency ‘Rcpp’
  trying URL 'https://cloud.r-project.org/bin/macosx/big-sur-arm64/contrib/4.5/Rcpp_1.0.14.tgz'
  trying URL 'https://cloud.r-project.org/bin/macosx/big-sur-arm64/contrib/4.5/V8_6.0.0.tgz'
  The downloaded binary packages are in
  /var/folders/46/wtyv3f9n7g1657wntpn0cl540000gn/T//RtmpMd0YSV/downloaded_packages


In the above output, there is no confirmation that the package
installation was successful. Instead we get a message that download
binary packages are to be found in a temporary directory, making it
seem as if the user has to do something with this? This is confusing.

Perhaps the messaging could be improved by not showing this tempir.
Instead install.packages() could end with a short success summary
like:

  Successfully installed packages 'V8', 'Rcpp' in
     /Library/Frameworks/R.framework/Versions/4.5-arm64/Resources/library

Or we could not show any directory at all. But at least the user knows
installation has succeeded.

While we're at it, the phrasing "trying URL" is a bit odd, both words
are redundant in this context. Maybe it can just be replaced by
"Downloading"

These are silly details but I think they make a difference especially
to novice users.