Feature request: ability to turn off automatic dependency search

[Benjamin Birnbaum]

Thanks for creating packrat -- it fills an important need for reliable dependency management in R.

As I've used packrat, I've had some problems with the automatic search for dependencies with packrat::init() and packrat::snapshot().  When I have those problems, the function throws an error and I'm completely unable to take snapshots.  I'd like it if there was an option to turn off the auto-discovery of dependencies.  For example, packrat::snapshot() would just look in lib/ for packages that weren't snapshotted and snapshot those.

Is this something you've considered?  I'm coming from the python world where the analog to packrat (virtualenv) doesn't do any kind of auto-search for dependencies.