package.bbclass: Adapt debugsrc code to workdir layout changes
With the change to WORKDIR layout, splutting PN and PV into two directories,
the debugsrc splutting code layout became suboptimal. This changes things to
include the information as it was before. Ideally this code would be written
to more generically support other layouts buts it not clear that the tools
would even support that right now so this is the best immediate fix.
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
