Issue warnings for missing py_modules?

Hello,

After more hours than I'd care to admit trying to figure out why setuptools was giving me a "no module found" error when using attr: in a config file, I finally tracked the issue down to a silly mistake: I'd specified py_modules instead of packages in the config file, and my code was in a package. The mistake stemmed from a copy-paste error: I'd copied an old configuration file because I didn't remember all the details of the format, and renamed things for my new project without remembering that py_modules would have to change to packages since the new project used a package (see https://github.com/pypa/setuptools/issues/3266).

The change I'd like to see it for the distutils code that checks modules to issue a warning when a listed module is not found, as opposed to silently ignoring this case, since that would have saved me a lot of time, and might also save time for others, on top of being quite reasonable behavior. The relevant code is lines 273-278 of command/build_py.py:

            # XXX perhaps we should also check for just .pyc files
            # (so greedy closed-source bastards can distribute Python
            # modules too)
            module_file = os.path.join(package_dir, module_base + ".py")
            if not self.check_module(module, module_file):
                continue

The equivalent code for finding packages raises an error on lines 189-195:

            if not os.path.exists(package_dir):
                raise DistutilsFileError(
                      "package directory '%s' does not exist" % package_dir)
            if not os.path.isdir(package_dir):
                raise DistutilsFileError(
                       "supposed package directory '%s' exists, "
                       "but is not a directory" % package_dir)

Since the comment in the modules code suggests the existence of .pyc files might be a reason for not being strict about modules existing, I think issuing a warning rather than an error is probably best to minimize what breaks. If this is something that maintainers here think is reasonable, I could put together a PR for raising a DistutilsFileWarning before continuing in the missing module case, which would include adding a DistutilsFileWarning class to errors.py.

Start in command/build_py.py around the module-checking code referenced in the issue, then inspect errors.py for the warning definitions. Done means a missing py_modules entry emits a DistutilsFileWarning instead of being silently ignored, while the existing continuation behavior remains intact.