Make revdep workflow reusable (#14014)

I thought we need to make this into an action but turns out Github
supports (somewhat limited) triggering workflows from other workflows.

This change allows triggering the workflow from another workflow by
passing the workflow inputs explicitely, while still allowing to run the
action manually.

This change needs to be merged separately since to trigger a workflow a
specific repo and revision have to be specified, thus to contribute an
action that triggers this action this must be accessible as `main`.
Therefore this is a separate PR with just this single change which will
allow testing the future PR against this workflow.

The workflow spec looks like this

```yaml
uses: ocaml/dune/.github/workflows/revdeps.yml@main
```

Unfortunately the `main` has to be specified, it can't be left out and
assumed to be the same branch. Likewise `ocaml/dune` can't be left out
to specify the current repo. Quite impractical overall.

Author: shonfeder

.github/workflows/revdeps.yml

@@ -1,6 +1,14 @@
 name: Reverse Dependencies
 
 on:
+  workflow_call:
+    inputs:
+      packages:
+        required: true
+        type: string
+      dune_version:
+        required: true
+        type: string
   workflow_dispatch:
     inputs:
       packages: