Move check_inits.py into Pants (pantsbuild#19919)

This change moves the pre-commit-hook (which ummm is that run in
CI?!?!?!) into an `experimental_test_shell_command`.

BUILD

@@ -22,3 +22,31 @@ remote_environment(
     name="buildgrid_remote",
     python_bootstrap_search_path=["<PATH>"],
 )
+
+files(
+    name="all-__init__.py-files",
+    sources=[
+        "**/__init__.py",
+        "!testprojects/**",
+        # These are explicit namespace packages
+        "!src/python/pants/__init__.py",
+        "!src/python/pants/testutil/__init__.py",
+    ],
+)
+
+# NB: This should be in `lint` when we implement `lint` in https://github.com/pantsbuild/pants/issues/17729
+experimental_test_shell_command(
+    name="checks-empty-init-files",
+    command="""
+        NONEMPTY_INITS=$(find . -type f -name "*.py" -size +0);
+
+        if [ -n "$NONEMPTY_INITS" ]; then
+            echo "All \\`__init__.py\\` file should be empty, but the following had content:";
+            echo "$NONEMPTY_INITS";
+            exit 1;
+        fi
+        exit 0;
+    """,
+    tools=["echo", "find"],
+    execution_dependencies=[":all-__init__.py-files"],
+)

build-support/githooks/pre-commit

@@ -35,6 +35,3 @@ if git diff "${MERGE_BASE}" --name-only | grep src/python/pants_release/generate
   echo "* Checking GitHub workflows generation"
   $_PANTS_PRE_COMMIT_BINARY run src/python/pants_release/generate_github_workflows.py -- --check
 fi
-
-echo "* Checking __init__.py files"
-$_PANTS_PRE_COMMIT_BINARY run build-support/bin/check_inits.py