Switch to the `dependent-issues` action. (#432)

This supports tracking PR-to-issue and issue-to-issue dependencies in
addition to PR-to-PR, and this seems likely to be increasingly important
as we have decisions being made via issues.

.github/workflows/dependent-issues.yaml

@@ -0,0 +1,31 @@
+# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+# Exceptions. See /LICENSE for license information.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+name: Dependent Issues
+
+on:
+  issues:
+    types:
+      - opened
+      - edited
+      - reopened
+  pull_request_target:
+    types:
+      - opened
+      - edited
+      - reopened
+  schedule:
+    - cron: '0 0 * * *' # daily
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: z0al/dependent-issues@v1
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+        with:
+          label: dependent
+          check_issues: on
+          keywords: depends on, blocked by

.github/workflows/dependent-pr.yaml

@@ -1,16 +0,0 @@
-# Part of the Carbon Language project, under the Apache License v2.0 with LLVM
-# Exceptions. See /LICENSE for license information.
-# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
-
-name: Dependencies
-
-on: [pull_request]
-
-jobs:
-  check_dependencies:
-    runs-on: ubuntu-latest
-    name: check PRs landed
-    steps:
-      - uses: gregsdennis/dependencies-action@main
-        env:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}