sd-bus: split introspection into the content creation and reply creation parts

sd-bus: split introspection into the content creation and reply creation parts

Just moving code around, in preparation to allow the content creation
part to be used in other places.

On the surface of things, introspect_path() should be in bus-introspect.c, but
introspect_path() uses many static helper functions in bus-objects.c, so moving
it would require all of them to be exposed, which is too much trouble.

test-bus-introspect is updated to actually write the closing bracket.