Pkg3: Runtime Configuration - Specification of Installed Packages?

First of all, really nice work!

As noted in the DifferentialEquations plan for modularization (https://github.com/JuliaDiffEq/DifferentialEquations.jl/issues/59), the only form of "conditional dependencies" that I really need is the ability to add/remove functionality at compile-time by knowing which packages are installed on the user's system. It seems like this can be done by the user specifying a flag in the Runtime Configuration file, but is there any way that this can be computed? As in, can this flag instead be determined a code which sees if packages X, Y, Z are installed, if so, allow me to use add functionality?

An example is that, at the top of the module, I'd want to:

``` julia
if SUNDIALS_INSTALLED
using Sundials
end
```

and then inside some higher level API functions have a branch which is allowed if `SUNDIALS_INSTALLED` (and error if not). If this is a possibility, then nothing would actually need to be changing at runtime, and thus the current conditional module problems that I have (which are not too difficult! I know there are more difficult conditional module problems which don't apply here) would be solved seamlessly to the user.