handle initially null driver when instantiating Mongoose for Rollup support

[vkarpov15]

Fix #12335

Summary

lib/index.js and lib/mongoose.js contain the significant changes here. The issue is that, because of how Rollup hoists imports when you don't set strictRequires: true, the setDriver() call in lib/mongoose.js will always run after the Mongoose singleton is initialized in lib/index.js. So the Mongoose constructor needs to handle null driver case, and lib/mongoose.js needs to call setDriver() to make sure the driver change propagates to the newly created Mongoose singleton.

In regular Node.js, that second setDriver() call is a no-op because setDriver() checks to see if new driver is === old driver.

On a related note, I also cleaned up some circular imports in lib/error/*.js. lib/error/index.js imports lib/error/cast.js and lib/error/cast.js imports lib/error/index.js. I was trying to see if cleaning up circular imports would help with Rollup support, but experimentation and further reading convinced me that it wasn't necessary for Rollup support. But still a nice to have since fixing the circular import is a bunch of one-liners.

Examples