Detects recursive `(include ...)` loops.

  $ dune build
  File "dune", line 1, characters 0-15:
  1 | (include a.inc)
      ^^^^^^^^^^^^^^^
  Error: Recursive inclusion of dune files detected:
  File a.inc is included from c.inc:1
     included from b.inc:1
  -> included from a.inc:1
  -> included from dune:1
  [1]
