Per-plugin LOGGER instances and standardized missing requirement reporting

In order to make it easier to debug stuff, and in order to de-centralize logging, let’s have every plugin get their own LOGGER instance.

This pull requests also includes a somehow-related feature of a `req_missing` function to standardize reporting missing requirements, and a not-at-all-related change to move Misaka out of a package, which it does not need right now.

(this PR is for code review reasons, and is targeted at @ralsina.)