(lang dune 2.8)
(using dune_site 0.1)

(package
  (name foo)
  (sites (share libs)))

(package
  (name foo.bar))

(generate_sites_module
 (module foo_sites)
 (sites foo))

(library
 (name foo)
 (package foo)
 (modules foo_sites))

% dune build -p foo.bar
File "dune", line 3, characters 8-11:
3 |  (sites foo))
            ^^^
Error: dune_site used outside a package