Avoid duplicates in the "Uses"-list

[bmaland]

E.g. Rltools both imports and suggests the xtable package

[bmaland]

Actually I'm not sure if this is a bug? Right now the page shows imports + depends in regular type, and suggested in italics. Maybe the UI for this part should be changed, since enhanced packages also should be displayed.

[hadley]

Good point. I don't think it's worth breaking up imports and depends, but suggests should be different, and maybe enhances should be in its own list.