We have two kinds of libweston users: internal and external. Weston, the
frontend, counts as an external user, and should not have access to libweston
private headers. The shell plugins are external users as well, because we
intend people to be able to write them. Renderers, backends, and some plugins
are internal users who will need access to private headers.

Create two different Meson dependency objects, one for each kind.

This makes it less likely to accidentally use a private header.

Screen-share is a Weston plugin and therefore counts as an external user, but
it needs the backend API to deliver input. Until we are comfortable exposing
public API for that purpose, let it use internal headers.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>