Replace os.path.sep.join(seq) with utils.PathJoin

This is a no-op change, but at least we concentrate the calls to path
joins into a single function.

A use in utils.FindFile is left as-is (don't want to raise exceptions
there, at least for now).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>