Semi-verbose flag that shows which files mypy analyzes

Sometimes (example: #6385) it's useful to be able to see exactly which files mypy is processing. Currently the only way is to use -v and grep stderr for the word "Parsing".

Maybe this is common enough that we could spare a new command-line flag? It would print all the files for which a BuildSource object is created, and all the files that are reached by following imports (with some indicator of which is which).

The order in which files are printed should reflect the order in which they are processed (not the order in which they are specified).

I wouldn't mind adding extra info for each file (e.g. whether its cache entry is used), as long as it remains a single line per file.

Presumably there's also value in printing certain failures, e.g. modules which can't be found but for which no error message is printed (e.g. through --follow-imports=silent). But I would stop here.

Comienza siguiendo la salida existente de -v que muestra "Parsing" en stderr; después, inspecciona dónde se crean los objetos BuildSource y dónde se siguen los imports. Se considera terminado cuando un nuevo flag de línea de comandos imprime una línea por cada file especificado directamente o importado, en el orden de procesamiento, con un indicador que distinga ambos casos; verifica la salida con ejecuciones representativas de mypy.