build: separate deps for int and ext libweston users

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.

Reduce the set of libraries these two dependency objects pull in, and link those libraries more explicitly when needed.

This MR is partially preparing for !230 (merged).

Closes: #219 (closed)