watch mode: don't kill ongoing actions if unrelated parts of the build change

It is possible at the moment to have a long-living executable —say, a web application server— handled by dune build watcher, so the exe is started as part of a dune action, and restarted when needed. More details can be seen in https://github.com/ocaml/dune/issues/7110.

One limitation though, is that dune will kill the long-living action every time a dependency changes in the build. Say we have this dune config:

(rule
 (alias melange_and_run_api_server)
 (deps
  (alias ./melange)
  (alias ./api_server))
 (action
  (echo "Building melange and running api server")))

(rule
 (alias api_server)
 (deps
  (:api_server server.bc))
 (action
  (run %{api_server})))

It would be ideal if the action in api_server could be left running when nothing in its dependency tree has changed.

Start by reading issue #7110 and the watch-mode behavior described here, then reproduce the example with the melange_and_run_api_server and api_server aliases. Trace how dependency changes cause long-running actions to stop. Done means an unrelated dependency change no longer kills the ongoing action, while changes in its own dependency tree still restart it.