Add script to identify backends not part of the pants distribution. (#…

…20548)

These mistakes are done over and over again, and goes undetected for too
long.

As a follow up, we could add a check mode and run during CI to catch
early. For now, it's merely a conveniency that can be run manually.

Example output (pre #20529):
```
❯ ./build-support/bin/check_orphaned_plugins.py
These are discovered backends, which are not included in the pantsbuild.pants distribution:
  *   (internal_plugins.releases - intentionally excluded)
  *   (internal_plugins.test_lockfile_fixtures - intentionally excluded)
  * pants.backend.build_files.fmt.ruff
  * pants.backend.experimental.typescript
  *   (pants_explorer.server - intentionally excluded)
```

---------

Co-authored-by: cburroughs <[email protected]>

build-support/bin/check_orphaned_plugins.py

@@ -0,0 +1,65 @@
+#!/usr/bin/env python
+# Copyright 2024 Pants project contributors (see CONTRIBUTORS.md).
+# Licensed under the Apache License, Version 2.0 (see LICENSE).
+
+from __future__ import annotations
+
+import json
+import subprocess
+from typing import cast
+
+NOT_DISTRIBUTED_BACKENDS = {
+    "internal_plugins.releases",
+    "internal_plugins.test_lockfile_fixtures",
+    "pants_explorer.server",
+}
+
+
+def main() -> None:
+    discovered_backends = get_backends()
+    distributed_backends = get_pants_binary_plugins()
+    orphaned_backends = discovered_backends - distributed_backends
+    if orphaned_backends:
+        print(
+            "These are discovered backends, which are not included in the pantsbuild.pants distribution:\n  * "
+            + "\n  * ".join(
+                f"  ({be} - intentionally excluded)" if be in NOT_DISTRIBUTED_BACKENDS else be
+                for be in sorted(orphaned_backends)
+            )
+        )
+
+
+def get_pants_binary_plugins() -> set[str]:
+    return {
+        plugin_path_to_backend(plugin)
+        for plugin in run_pants("peek", "src/python/pants/bin:plugins")[0]["dependencies_raw"]
+    }
+
+
+def plugin_path_to_backend(plugin: str) -> str:
+    return plugin.replace("src/python/", "").replace("/", ".")
+
+
+def get_backends() -> set[str]:
+    return set(get_help_info()["name_to_backend_help_info"])
+
+
+def get_help_info() -> dict:
+    return run_pants("help-all")
+
+
+def run_pants(*args: str) -> dict:
+    return cast(
+        "dict",
+        json.loads(
+            subprocess.run(
+                ["pants", *args],
+                stdout=subprocess.PIPE,
+                check=True,
+            ).stdout.decode()
+        ),
+    )
+
+
+if __name__ == "__main__":
+    main()