The proxy API is the one that we would like to expose for applications
and the other API is used internally when implementing modules or
factories.
The current pw_core object is really a context for all objects so
name it that way. It also makes it possible to rename pw_core_proxy
to pw_proxy later.
meson < 0.46 does not allow two targets to share the same name;
meson >= 0.46 relaxes that requirement, but I see no reason to require
a new version of meson just for this