package infra: add patch-dependencies

Some packages need to vampirise files from one or more other packages.
This is the case, for example, of the Linux kernel and its /extensions/.

Add a new type of dependencies, that are guaranteed to be extracted and
patched before a package is patched.

[Thomas: remove <pkg>-show-build-depends and <pkg>-show-patch-depends,
since they don't seem to really be necessary and very useful.]

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>