package alternatives in Pkg3?

I wonder whether we want Pkg3 to contain something like [Debian alternatives](https://wiki.debian.org/DebianAlternatives): a package A can depend on package B *or* package C *or* package D.

For example, the Plots.jl package might want to depend on having at least one plotting package installed (cc @tbreloff).

Or, now that FFTW.jl is being split off into its own package, and that package in turn will probably depend on an AbstractFFTs package (JuliaMath/FFTW.jl#2), other packages may want to depend on having *some* package installed that implements FFTs following the AbstractFFT interface (e.g. FFTW, or a native Julia FFT, or FFTPACK, or MKL, etcetera).