Add check job for upstream URLs on imported repos

Add a check job to require upstream URLs to use a scheme
that we are confident won't cause connection errors during
the import.

Add a tox configuration for this new job, as well as the
existing alphabetical check script, to make it easier
for developers to run those tests locally just by running
"tox".

Change-Id: I894ac0f8afb234404bb6e80e64b90aa25e052f10
Merge "Add check job for upstream URLs on imported repos"