Start making headers private

With libphosh-rs users making it into distributions we want to make libphosh stable ABI rather sooner than later. Most of the classes we export are currently not used by anyone so instead of exporting new classes introduced in 0.44 lets rather make them private.

This will make the generated docs less useful as not all classes show up but we can generate a second gir for that if needed.

This commit can server as a template in case someone wants to help reducing the API surface. The pattern is

• move headers to libphosh_private_headers
• remove from libphosh.h

This will unbreak the libphosh-rs build