misc: Refactor scan process by splitting single function

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.

backend/exceptions/socket_exceptions.py

@@ -0,0 +1 @@
+class ScanStoppedException(Exception): ...