build_package crashes on external links like \link[package:topic]{other name}

[renozao]

Hi Hadley,

I just discovered this yet another great tool from you!
I tried it on a package but got this error:

Generating registry.html
Error in find.package(package, lib.loc, verbose = verbose) : 
  there is no package called ‘registry:regobj’

It seems it breaks on the link \code{\link[registry:regobj]{registry}} which is supposed to link to the topic regobj of the package registry.
I will fork and try to provide a patch for this.

Thanks