[Dependency] Add a field to filter platforms where a dependency is used

[giordano]

I think it could be useful to have a field to the Dependency struct to filter the platforms where the package will actually be loaded. We can't have platform-dependent dependencies because Pkg doesn't allow it, but we can at least avoid loading them if not necessary, even if they are mostly no-op. For example, GTK3_jll on macOS and Windows could not load the whole X11 stack, and NativeFileDialog_jll could not load GTK3_jll on the same platforms.

CC: @fingolfin

[giordano]

Fixed by #176 and JuliaPackaging/BinaryBuilder.jl#1129