lib: yang: use common yang_ctx_new_setup()

lib: yang: use common yang_ctx_new_setup()

After creating a libyang context, we need to hook up our callback to use
embedded built-in modules.  I hadn't added this to the yang translator
code.

Also, ly_ctx_new fails if the search directory doesn't exist.  Since
that's not a hard error for us, work around that and ignore inaccessible
YANG_MODELS_DIR.  (This is needed for snap packages.)

Signed-off-by: David Lamparter <equinox@diac24.net>