use dune_site module to find inputs

this packages the inputs as part of the build and exposes the directory
to find them via a generated module. this way, inputs are a resource of
the library rather than an input.

this means that `dune exec aoc` and `dune utop` will always find the
file regardless of where they are invoked, which makes developing in
emacs simpler :)

dune-project

@@ -15,11 +15,14 @@
 
 (documentation https://url/to/documentation)
 
+(using dune_site 0.1)
+
 (package
  (name aoc)
  (synopsis "A short synopsis")
  (description "A longer description")
  (depends ocaml dune)
+ (sites (lib inputs))
  (tags
   (topics "to describe" your project)))