tools: Use root-based absolute import for tools.lib, etc.

Mypy can’t follow absolute imports based on directories other than the
root.  This was hiding some type errors due to ignore_missing_imports.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/build-docs

@@ -1,16 +1,17 @@
 #!/usr/bin/env python3
 
-# check for the venv
-from lib import sanity_check
-
-sanity_check.check_venv(__file__)
-
 import argparse
 import os
 import sys
 
 ZULIP_PATH = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
 sys.path.append(ZULIP_PATH)
+
+# check for the venv
+from tools.lib import sanity_check
+
+sanity_check.check_venv(__file__)
+
 from scripts.lib.zulip_tools import ENDC, WARNING, run