Update tox testenv 'vendor' to use cog to automatically update/check *.extern

Author: abravalheri

tox.ini

@@ -69,12 +69,16 @@ pass_env = *
 commands =
 	python tools/finalize.py
 
-[testenv:vendor]
+[testenv:{vendor,check-extern}]
 skip_install = True
+allowlist_externals = sh
 deps =
 	path
+	cogapp
 commands =
-	python -m tools.vendored
+	vendor: python -m tools.vendored
+	vendor: sh -c "git grep -l -F '\[\[\[cog' | xargs cog -I {toxinidir} -r"  # update `*.extern`
+	check-extern: sh -c "git grep -l -F '\[\[\[cog' | xargs cog -I {toxinidir} --check"
 
 [testenv:generate-validation-code]
 skip_install = True