Indeed, but the type of jobs generated by these types of projects are hardly pointless ditch digging.
The benefit from additional discoveries is already factored into the cost of conventional solar.
But only if the companies are Japanese. Otherwise Japan is not only helping grow another countries GDP they're also helping them gain a further technology lead. Doing everything 'in house' is probably more inefficient in the short term, but might offer more long term benefits.
In this case, certainly the whole world would benefit even if a better solar energy technology (conventional or outer space) were invented. The polio vaccine helped the whole world; the reason that most of the revenue from it came to the USA is that the rest of the world thought that having those shots was more important than giving up their money.
And in economic terms, if tilting the economies of scale makes Japan a better manufacturer of solar power tech, then the law of comparative advantage allows other countries to benefit from supplying the goods that Japan will shift away from as it moves resources to the solar tech.
The benefit from additional discoveries is already factored into the cost of conventional solar. The companies building it have business plans that recognize the development work they're doing, and charge prices accordingly. And that's part of why it's cheap (relative to this proposal) and getting cheaper.