feat(dep): source header edges from compiler depfiles

Parses -MMD -MF output, unfolding line continuations, so the include set comes
from the compiler rather than from hand-maintained declarations.

lib/dep.ml

@@ -0,0 +1,23 @@
+let of_file path =
+  match Fs.read path with
+  | exception Sys_error _ -> None
+  | raw ->
+      let b = Buffer.create (String.length raw) in
+      let n = String.length raw in
+      let i = ref 0 in
+      while !i < n do
+        if raw.[!i] = '\\' && !i + 1 < n && raw.[!i + 1] = '\n' then (Buffer.add_char b ' '; i := !i + 2)
+        else if raw.[!i] = '\n' then (Buffer.add_char b ' '; incr i)
+        else (Buffer.add_char b raw.[!i]; incr i)
+      done;
+      let flat = Buffer.contents b in
+      match String.index_opt flat ':' with
+      | None -> None
+      | Some c ->
+          let rhs = String.sub flat (c + 1) (String.length flat - c - 1) in
+          let parts =
+            String.split_on_char ' ' rhs
+            |> List.map String.trim
+            |> List.filter (fun s -> s <> "" && s <> "\\")
+          in
+          Some parts