Create dependency on "puredata-core" rather than "puredata"

AS of now, dh_puredata_substvar creates dependencies on the "puredata" metapackage, e.g.

Depends: puredata | pd | puredata64 | pd64

This is suboptimal, as it will pull in the X11 stack (via puredata-gui).

I think it would be better to create a dependency on puredata-core | puredata64-core | ... instead. The full puredata stack could be pulled in via Recommends (so the "normal" user who installs an external, will get a useable Pd environment)