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.

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):