You should not have .js in your require. It should be:
require(["object/extend"], function (extend){
});
When RequireJS sees a dot in the module name it appears to assume that this is a full URL and does not process the baseUrl, etc. Note, however, that you can still use a dot in the module name if you've defined it as a path mapping:
require.config({
paths: {
'a.b': 'libraries/something/a.b'
}
});
and then later require it:
require(['a.b'], function (ab){
});