Use of architecture-specifying colons in dependency lists

[Julian Gilbey <julian@d-and-j.net>]

Hello,

I have noticed that packages are using architecture specifiers in
their dependency information, for example dh-python depends on
'python3:any (>= 3.3.2-2~)'.  However, there is no mention of this
syntax that I could find in the current policy.

I would propose a patch, but I don't know what the correct semantics
should be (though I could have a decent guess!).

   Julian