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 :)

src/dune

@@ -1,4 +1,12 @@
 (library
  (name aoc)
- (libraries stdio re)
+ (libraries stdio re dune-site)
  (preprocess (pps ppx_regexp)))
+
+(install
+ (section (site (aoc inputs)))
+ (files (glob_files ../inputs/*)))
+
+(generate_sites_module
+ (module inputs)
+ (sites aoc))