This change mainly refactors the `scan_platforms` function, moving part
of its logic to `_identify_platform`, `_identify_firmware`, and
`_identify_rom`.
The logic is simpler this way, each smaller function returns `ScanStats`
that can be merged by the caller, and it simplifies future performance
improvements.
The current implementation for some of the database handlers, where the
same method is used to retrieve either a single entity (when an `id` is
passed), a list of entities, or `None`, makes the typing and overall
design more complex.
This change simplifies database handlers, by having two separate methods
where appropiate:
* A method that receives an `id`, and returns either an entity, or `None`.
* A method that optionally receives filters, and returns (depending on
the current handler implementation) a list of entities, or a `Select`
object that allows chaining more SQLAlchemy operations.
