title: devtools::install_git fails to install dependencies of packages in Depends or Imports
body: <p>I have a package that depends on the package <code>extrafont</code>.  If extrafont and its dependency <code>Rttf2pt1</code> aren't present on the user's system, installation of my package fails.  I have <code>extrafont</code> as "Depends" in my package's DESCRIPTION file.  When I run <code>devtools::install_git()</code> to install my package from a URL, the output terminates with:</p>
  
  <pre><code>** preparing package for lazy loading
  Error in loadNamespace(j &lt;- i[[1L]], c(lib.loc, .libPaths()), versionCheck = vI[[j]]) : 
    there is no package called 'Rttf2pt1'
  Error : package 'extrafont' could not be loaded
  ERROR: lazy loading failed for package 'tntpr'
  * removing 'C:/Users/SFirke/Documents/R/win-library/3.3/tntpr'
  </code></pre>
  
  <p>Looks like extrafont installs okay, but doesn't work because <code>Rttf2pt1</code> is missing.  When I add <code>Rttf2pt1</code> to the Depends list in my DESCRIPTION file, then the installation succeeds, installing both <code>extrafont</code> and <code>Rttf2pt1</code> and my package.</p>
  
  <p>Why do I need to put <code>Rttf2pt1</code> in my Depends list?  It's present on the "Imports" list in the DESCRIPTION file from the <code>extrafont</code> package:</p>
  
  <pre><code>Depends:
      R (&gt;= 2.15)
  Imports:
      extrafontdb,
      grDevices,
      utils,
      Rttf2pt1
  </code></pre>
  
  <p><strong>Additional info</strong></p>
  
  <p>When I run <code>devtools::install_git("https://myurl.com/tntpr.git", dependencies = TRUE)</code>, I get the following output.  It installs dplyr from GitHub, then extrafonts, then fails loading my package:</p>
  
  <pre><code>Installing tntpr
  Downloading GitHub repo hadley/dplyr@master
  from URL https://api.github.com/repos/hadley/dplyr/zipball/master
  Installing dplyr
  "C:/PROGRA~1/R/R-33~1.2/bin/x64/R" --no-site-file --no-environ --no-save --no-restore --quiet CMD INSTALL  \
    "C:/Users/SFirke/AppData/Local/Temp/RtmpMlRSSR/devtools3dfc4e39620/hadley-dplyr-5902277" --library="C:/Users/SFirke/Documents/R/win-library/3.3" --install-tests 
  
  * installing *source* package 'dplyr' ...
  ** libs
  
  *** arch - i386
  C:/RBuildTools/3.4/mingw_32/bin/g++  -I"C:/PROGRA~1/R/R-33~1.2/include" -DNDEBUG -I../inst/include -DCOMPILING_DPLYR   -I"C:/Users/SFirke/Documents/R/win-library/3.3/Rcpp/include" -I"C:/Users/SFirke/Documents/R/win-library/3.3/BH/include" -I"C:/Users/SFirke/Documents/R/win-library/3.3/bindrcpp/include" -I"C:/Users/SFirke/Documents/R/win-library/3.3/plogr/include" -I"d:/Compiler/gcc-4.9.3/local330/include"     -O2 -Wall  -mtune=core2 -c RcppExports.cpp -o RcppExports.o
  &lt;--- lots more lines like this ^^^^^ ---&gt;
  ** R
  ** data
  *** moving datasets to lazyload DB
  ** inst
  ** tests
  ** preparing package for lazy loading
  ** help
  *** installing help indices
  ** building package indices
  ** installing vignettes
  ** testing if installed package can be loaded
  *** arch - i386
  *** arch - x64
  * DONE (dplyr)
  Installing 1 package: extrafont
  Installing package into ‘C:/Users/SFirke/Documents/R/win-library/3.3’
  (as ‘lib’ is unspecified)
  trying URL 'https://cran.rstudio.com/bin/windows/contrib/3.3/extrafont_0.17.zip'
  Content type 'application/zip' length 34323 bytes (33 KB)
  downloaded 33 KB
  
  package ‘extrafont’ successfully unpacked and MD5 sums checked
  
  The downloaded binary packages are in
      C:\Users\SFirke\AppData\Local\Temp\RtmpMlRSSR\downloaded_packages
  "C:/PROGRA~1/R/R-33~1.2/bin/x64/R" --no-site-file --no-environ --no-save --no-restore --quiet CMD INSTALL "C:/Users/SFirke/AppData/Local/Temp/RtmpMlRSSR/file3dfc4a973a21"  \
    --library="C:/Users/SFirke/Documents/R/win-library/3.3" --install-tests 
  
  * installing *source* package 'tntpr' ...
  ** R
  ** data
  *** moving datasets to lazyload DB
  ** inst
  ** tests
  ** preparing package for lazy loading
  Error in loadNamespace(j &lt;- i[[1L]], c(lib.loc, .libPaths()), versionCheck = vI[[j]]) : 
    there is no package called 'Rttf2pt1'
  Error : package 'extrafont' could not be loaded
  ERROR: lazy loading failed for package 'tntpr'
  * removing 'C:/Users/SFirke/Documents/R/win-library/3.3/tntpr'
  Error: Command failed (1)
  </code></pre>