title: devtools does not check if dependencies are already installed
body: <p>I am using <code>devtools::install_url(&quot;pkg_url&quot;, dependencies = TRUE, upgrade = &quot;always&quot;)</code> to install specific versions of packages in a singularity build file. However I noticed during the installation procedure that each time it is run for each package, it installs many of the same dependencies over again, even though this was installed for one of the previous packages (same versions). I am not 100% sure how <code>devtools::install_url()</code> works, but this is making the installation runtime very long. Is there a way to make it check for the presence of these packages before trying to install them?</p>