[Rd] Small patch to packages.R

[Chris Masters]

Very small patch to packages.R

install.packages on an internal package repository that worked with 2.9
failed in 2.10

Problem:
> available.packages(contriburl="http://url.to.my/repo")
Error in f(res) : invalid subscript type 'list'

This occurred because the PACKAGES file had no dependency on R in the
Depends field on any of the packages. Got around it by adding "Depends: R
(>=2.7.0)" to the packages but it was still a regression from 2.9 behaviour.

Cheers

Chris