Make it possible to use `modules` or `packages` configurations with `files`

[alxroyer]

Feature

Make it possible to use modules or packages configurations with files.

Pitch

Currently, when using modules or packages configurations with files, we get the following error:

usage: mypy [-h] [-v] [-V] [more options; see below]
            [-m MODULE] [-p PACKAGE] [-c PROGRAM_TEXT] [files ...]
mypy: error: May only specify one of: module/package, files, or command.

I did not see any mention of such restriction in the documentation.

Nevertheless, it could be useful to check in a single mypy execution:

• the core packages with modules or packages, which configurations seem to handle correctly pkgutil-style namespace packages (see Add support for pkgutil-style namespace packages #9393),
• other standalone scripts (tools, tests, ...) with files.