title: tmPlot is in namespace, but its dependencies are not found
body: <p>I have a package which uses the <code>tmPlot</code> function from <code>treemap</code>, but when I try to use the function, it throws an error that one of its dependencies isn't loaded:</p>
  
  <pre><code>Error in tmPlot(data, index = index, vSize = vSize) : 
    could not find function "brewer.pal"
  </code></pre>
  
  <p>The dependency is installed and in the namespace.</p>
  
  <p>This question has a little bit of setup, being a package problem, but I've tried to make it as minimal as possible:</p>
  
  <p><strong>Ensure you have <code>treemap</code> (and all its dependencies) installed.</strong></p>
  
  <p><strong>I've made a directory called 'anRpackage'. Inside it is a folder ('R') and a DESCRIPTION file with the following text:</strong></p>
  
  <pre><code>Package: anRpackage
  Title: What the package does (short line)
  Version: 1.0
  Author: Who wrote it
  Maintainer: Who to complain to &lt;yourfault@somewhere.net&gt;
  Description: More about what it does (maybe more than one line)
  License: What license is it under?
  Imports:
      treemap
  Collate:
      'maketree.R'
  </code></pre>
  
  <p><strong>Inside the R/ folder is a single R file called 'maketree.R'. Its contents are:</strong></p>
  
  <pre><code>#' maketree
  #' 
  #' @importFrom treemap tmPlot
  #' @export maketree
  
  maketree &lt;-
  function(data, index, vSize){
    tmPlot(data, index=index, vSize=vSize)
  }
  </code></pre>
  
  <p><strong>Assuming you're in the directory above 'anRpackage', run the following script:</strong></p>
  
  <pre><code>library(roxygen2)
  roxygenise("anRpackage/")
  
  library(devtools)
  
  build("anRpackage")
  install("anRpackage")
  </code></pre>
  
  <p><strong>Restart R (preferably with --vanilla) and run the following:</strong></p>
  
  <pre><code>library(anRpackage)
  
  data(mtcars)
  maketree(mtcars, "cyl", "mpg")
  </code></pre>
  
  <p>You should get the error I described right at the beginning. Why does this happen? <code>RColorBrewer</code> is listed as Depends for <code>treemap</code>, so it should be be automatically imported, should it not?</p>