stop recording deps in Action_builder.contents

PR ocaml#9552 changed the semantics of Action_builder.contents so that it
records a dependency. The Coq rules were relying on the previous
behaviour of not recording a dependency in order to generate a build
plan. This caused issue ocaml#10149 to appear.

This PR fixes ocaml#10149 by removing the dependency being recorded in
Action_builder.contents matching the previous semantics.

Signed-off-by: Ali Caglayan <alizter@gmail.com>

doc/changes/10446.md

@@ -0,0 +1,2 @@
+- Fix a bug where Coq projects were being rebuilt from scratch each time the
+  dependency graph changed. (#10446, fixes #10149, @alizter) 

src/dune_rules/action_builder.ml

@@ -37,12 +37,7 @@ let paths ps = deps (Dep.Set.of_files ps)
 let path_set ps = deps (Dep.Set.of_files_set ps)
 let dyn_paths paths = dyn_deps (paths >>| fun (x, paths) -> x, Dep.Set.of_files paths)
 let dyn_paths_unit paths = dyn_deps (paths >>| fun paths -> (), Dep.Set.of_files paths)
-
-let contents p =
-  let* () = Dep.file p |> Dep.Set.singleton |> Build_system.record_deps in
-  of_memo (Build_system.read_file p)
-;;
-
+let contents p = of_memo (Build_system.read_file p)
 let lines_of p = contents p >>| String.split_lines
 
 let read_sexp p =

test/blackbox-tests/test-cases/coq/no-rebuild-on-dep-change.t

@@ -11,8 +11,7 @@
 This test makes sure that a full rebuild is not triggered when the output of
 coqdep is changed.
 
-Currently this is the case and a bug:
+This is as expected:
   $ dune build --display=short
         coqdep .bug.theory.d
-          coqc root.{glob,vo}
           coqc leaf.{glob,vo}