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

[Martin Pool]

>  >> Excellent!  Patch against 1.2rc1 and tested with bzr-1.1 attached.
>
>
> bb:resubmit
>
>  I'm sure I don't need such path modification under Windows.

But it seems like it would be harmless?  It will probably be the same
as a directory that's already on the path?

-- 
Martin