Fix error in payload generated by ImportmapGenerator

[izaguirrejoe]

The current implementation isn't working correctly, self.targets is already an array. This removes the extra brackets, adds pytest as a dev dependency, and adds missing tests for ImportmapGenerator.

[davegaeddert]

Interesting. I will try to take a look this week — it definitely looks like you're right at a glance, I just can't imagine how this could have been wrong unless something changed!