extensions: ignore "sub-options" when looking for extensions
config suboptions are separated by ":" (see the path one for example). So we
dont want to confuse these config with actual extensions.
We don't have extensions sub option yet, but I am about to introduce one for
making sure an extensions can load. So lets level the floor first.
Differential Revision: https://phab.mercurial-scm.org/D11819