metadata: handle target build depends together with host build depends

Target build depends are similar to host build depends in that they refer
to source packages rather than binary packages. Therefore, it makes sense
to handle them together, rather than putting them in a list together with
runtime depends and trying to figure out if the entries refer to source or
to binary packages afterwards.

This does lead to PKG_BUILD_DEPENDS entries referring to binary package
names not working anymore, which requires some fixes in the package
repositories.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>