require_once instead of depending on the return value of include_once

require_once instead of depending on the return value of include_once

return value from include_once is unreliable, it could be the value
of a 'return;' statement in the included file, or it could be false
when the file wasn't readable. This was breaking deployments because
one of the extensions had "return;" which caused include_once to
return a falsy value.

Change-Id: I48b9a55d5f9e85efe515d87b56b60ee71f939842