[merge] look for plugins in arch-independent site directory (was Re: Bundling more plugins into bzrtools)

[Ian Clatworthy]

Ian Clatworthy has voted approve.
Status is now: Semi-approved
Comment:
Going through the history of this patch, it seems to have been approved 
in earlier forms several times. There is certainly potential for doing 
more than this but it's a step forward so I think we ought to merge it.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C47CACCAA.3060700%40ukr.net%3E