Split github_tools into its own bazel repo. (#3632)

The pip dependencies in github_tools are the reason the
MODULE.bazel.lock is platform-dependent. Following complaints about the
platform-dependence, split apart github_tools from the rest of the bazel
repo and make it not track the lockfile: while there's an incremental
safety risk due to not tracking checksums, it's unlikely the tools there
would ever be part of a Carbon release process. If we eventually add
Python tools that need pip to the release, it might be desirable to go
back to re-unify the bazezl repos.

This does make running pr_comments incrementally more inconvenient
because a "bazel run" needs to be run from the github_tools subdir.

As a consequence of separating the dependency, this means tests will not
be continuously run in github_tools. They're now a separate repo, and we
cannot add a dependency without restoring the platform-dependent issue.
I think pr_comments is sufficiently low value and unchanging that it is
not worth building separate CI for it.

Cleans up some legacy references to third_party/llvm-project, since now
github_tools needs to be added to the main bazelignore.

.bazelignore

@@ -4,6 +4,6 @@
 
 # Carbon creates some non-standard bazel directories, so ignore them.
 bazel-carbon-lang
-third_party/llvm-project/libcxx/utils/google-benchmark
-third_party/llvm-project/utils/bazel/examples
-third_party/llvm-project/utils/bazel/llvm-project-overlay
+
+# See github_tools/MODULE.bazel.
+github_tools

.gitignore

@@ -5,6 +5,10 @@
 # Directories and symlinks generated by bazel.
 /bazel-*
 
+# See github_tools/MODULE.bazel.
+/github_tools/bazel-*
+/github_tools/MODULE.bazel.lock
+
 # Directories created by python.
 **/__pycache__/
 

MODULE.bazel

@@ -167,12 +167,3 @@ python.toolchain(
     python_version = "3.11",
 )
 use_repo(python, "python_versions")
-
-# Create a central repo that knows about the pip dependencies.
-pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")
-pip.parse(
-    hub_name = "py_deps",
-    python_version = "3.11",
-    requirements_lock = "//github_tools:requirements.txt",
-)
-use_repo(pip, "py_deps")

github_tools/MODULE.bazel

@@ -0,0 +1,30 @@
+# 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
+
+"""Bazel modules.
+
+This is set up as a separate bazel repo from the main Carbon directory in order
+to split up the pip dependency, which makes MODULE.bazel.lock
+platform-dependent. We don't commit the MODULE.bazel.lock for github_tools
+because it's low-value, and would shift the platform dependence costs.
+"""
+
+module(name = "github_tools")
+
+bazel_dep(name = "rules_python", version = "0.27.1")
+
+python = use_extension("@rules_python//python/extensions:python.bzl", "python")
+python.toolchain(
+    python_version = "3.11",
+)
+use_repo(python, "python_versions")
+
+# Create a central repo that knows about the pip dependencies.
+pip = use_extension("@rules_python//python/extensions:pip.bzl", "pip")
+pip.parse(
+    hub_name = "py_deps",
+    python_version = "3.11",
+    requirements_lock = "//:requirements.txt",
+)
+use_repo(pip, "py_deps")

github_tools/WORKSPACE

@@ -0,0 +1,5 @@
+# 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
+
+workspace(name = "github_tools")

github_tools/github_helpers_test.py

@@ -9,7 +9,7 @@ SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 import unittest
 from unittest import mock
 
-from github_tools import github_helpers
+import github_helpers
 
 
 _TEST_QUERY = """

github_tools/pr_comments_test.py

@@ -10,8 +10,8 @@ import os
 import unittest
 from unittest import mock
 
-from github_tools import github_helpers
-from github_tools import pr_comments
+import github_helpers
+import pr_comments
 
 
 class TestPRComments(unittest.TestCase):

github_tools/requirements.txt

@@ -2,7 +2,7 @@
 # This file is autogenerated by pip-compile with Python 3.11
 # by the following command:
 #
-#    bazel run //github_tools:requirements.update
+#    bazel run //:requirements.update
 #
 certifi==2023.11.17 \
     --hash=sha256:9b469f3a900bf28dc19b8cfbf8019bf47f7fdd1a65a1d4ffb98fc14166beb4d1 \
@@ -103,7 +103,7 @@ charset-normalizer==3.3.2 \
 gql==2.0.0 \
     --hash=sha256:35032ddd4bfe6b8f3169f806b022168932385d751eacc5c5f7122e0b3f4d6b88 \
     --hash=sha256:fe8d3a08047f77362ddfcfddba7cae377da2dd66f5e61c59820419c9283d4fb5
-    # via -r github_tools/requirements.in
+    # via -r requirements.in
 graphql-core==2.3.2 \
     --hash=sha256:44c9bac4514e5e30c5a595fac8e3c76c1975cae14db215e8174c7fe995825bad \
     --hash=sha256:aac46a9ac524c9855910c14c48fc5d60474def7f99fd10245e76608eba7af746