init() fails after it detects a dependency to an unavailable package

init() fails after it detects a dependency to a package that is not available.

Steps to reproduce:

Create a project directory with a single R file that contains the following:

library(smeg)
smeg::smog()

Run packrat::init() on the directory

Initializing packrat project in directory:

• "/tmp/smeg"
  Error in if (result$source == "unknown" && fallback.ok) { :
  missing value where TRUE/FALSE needed
  In addition: Warning message:
  In FUN(X[[i]], ...) : Package 'smeg' not available in repository or locally

Tested with packrat at 145364480ae13dd1b65c21fd871819526caef59f:

sessionInfo()
R version 3.2.0 (2015-04-16)
 Platform: x86_64-unknown-linux-gnu (64-bit)
 Running under: Ubuntu precise (12.04.4 LTS)
 
 locale:
 [1] LC_CTYPE=en_US.ISO-8859-1 LC_NUMERIC=C LC_TIME=en_US.ISO-8859-1
[4] LC_COLLATE=en_US.ISO-8859-1 LC_MONETARY=en_US.ISO-8859-1 LC_MESSAGES=en_US.ISO-8859-1
[7] LC_PAPER=en_US.ISO-8859-1 LC_NAME=C LC_ADDRESS=C
[10] LC_TELEPHONE=C LC_MEASUREMENT=en_US.ISO-8859-1 LC_IDENTIFICATION=C
 
attached base packages:
 [1] stats graphics grDevices utils datasets methods base
 
 loaded via a namespace (and not attached):
 [1] tools_3.2.0 packrat_0.4.3-25

Start by reproducing the failure with packrat::init() on a directory containing an R file that calls library(smeg) and smeg::smog(). Inspect the dependency detection path reached by init() and the reported source == "unknown" condition. Done means unavailable packages no longer cause the missing-value error during initialization.