title: Only download sources of a package and all dependencies
body: <p>I am wondering if there's a way to use <code>install.packages()</code> or other related functions to do the following: <em>only download the sources</em> (i.e. <code>tar.gz</code> files) of the specified packages and all their dependencies into a specified folder (on Windows).</p>
  
  <p>One reason to do this is: say I have a Linux account that is not enabled for internet access. In order to install the packages on the Linux machine, I would first download all the needed sources on my Windows machine, then ftp them over to the Linux machine, and install them on the Linux machine using </p>
  
  <pre><code>  install.packages('/home/me/R/Packages/blah.tar.gz', repos = NULL)
  </code></pre>