Gracefully handle deleted packages in analysis

Previously, the analysis phase didn't expect any package deletions. So,
we'd fail with the error "impossible". But, with the new opam-repository
archival process, we allow for deletions, even though only in the
special case of a package being archived.

lib/analyse.ml

@@ -245,9 +245,12 @@ module Analysis = struct
     Fpath.v "packages" // name // (name^"."^version) // "opam"
 
   let add_package_data ~dir (pkg, kind) packages =
-    match packages with
-    | Error _ as err -> Lwt.return err
-    | Ok packages ->
+    match (packages, kind) with
+    | Error _ as err, _ ->
+      Lwt.return err
+    | Ok packages, Deleted ->
+      Lwt.return_ok ((pkg, {kind; has_tests = false}) :: packages)
+    | Ok packages, _ ->
       let open Lwt_result.Syntax in
       let path = Fpath.to_string (package_to_path pkg) in
       let* content = get_opam ~cwd:dir path |> Lwt_result.map_error (fun _ -> `Msg "impossible") in

test/analyse.t

@@ -109,7 +109,11 @@ Test deleting existing packages works
   * a-1.0.0.1 removed (HEAD -> new-branch-2)
   * a-1 (tag: initial-state, new-branch-1, master)
   $ opam-repo-ci-local --repo="." --branch=new-branch-2 --analyse-only --no-web-server
-  Error "impossible"
+  {
+    "packages": [
+      [ "a-1.0.0.1", { "kind": [ "Deleted" ], "has_tests": false } ]
+    ]
+  }
 
 Clean up the build cache