A dependency alias (created in populated_depend_aliases) is some name (such as
java) and some associated run and build time dependencies, specified like this:

    alias_build java openjdk-6-jdk
    alias_build java libgcj9-dev
    alias_run java openjdk-6-jre

So when cran2deb needs to use the 'java' build dependency, it will add
"openjdk-6-jdk, libgcj9-dev" to the Build-Depends:. alias_run deals with
Depends: only.
Since in Debian you cannot Build-Depend: upon build-essential, there is a
special 'ignore' dependency alias (this can be handy for dropping unnecessary
system requirements)

    alias_build ignore build-essential

populate_forcedep contains like:

    force java rJava

which forces the R package rJava to use the dependency alias 'java'. This is
for cases where there is no SystemRequirement.

Finally, populate_sysreq has lines like:

   sysreq quantlib quantlib%

This says, whenever a part of a SystemRequirement matches the SQL LIKE pattern
'quantlib%', use the dependency alias. SystemRequirements are converted to
lower case and messed around with; details are in R/debcontrol.R in the
sysreqs_as_debian function. R/debcontrol.R contains pretty much all of the code
for dependencies (the database interface code is in R/db.R).