Add setuptools integration

This proved simpler because I realized something important: Most of the
code that the old integration was using was in fact doing nothing of
value. Since we can't meaningfully allow users to use command-line
options as parameters to `python setup.py flake8`, we just remove the
work that we do to set up those attributes and parse them.

flake8/main/setuptools_command.py

@@ -0,0 +1,77 @@
+"""The logic for Flake8's integration with setuptools."""
+import os
+
+import setuptools
+
+from flake8.main import application as app
+
+
+class Flake8(setuptools.Command):
+    """Run Flake8 via setuptools/distutils for registered modules."""
+
+    description = 'Run Flake8 on modules registered in setup.py'
+    # NOTE(sigmavirus24): If we populated this with a list of tuples, users
+    # could do something like ``python setup.py flake8 --ignore=E123,E234``
+    # but we would have to redefine it and we can't define it dynamically.
+    # Since I refuse to copy-and-paste the options here or maintain two lists
+    # of options, and since this will break when users use plugins that
+    # provide command-line options, we are leaving this empty. If users want
+    # to configure this command, they can do so through config files.
+    user_options = []
+
+    def initialize_options(self):
+        """Override this method to initialize our application."""
+        pass
+
+    def finalize_options(self):
+        """Override this to parse the parameters."""
+        pass
+
+    def package_files(self):
+        """Collect the files/dirs included in the registered modules."""
+        seen_package_directories = ()
+        directories = self.distribution.package_dir or {}
+        empty_directory_exists = '' in directories
+        packages = self.distribution.packages or []
+        for package in packages:
+            package_directory = package
+            if package in directories:
+                package_directory = directories[package]
+            elif empty_directory_exists:
+                package_directory = os.path.join(directories[''],
+                                                 package_directory)
+
+            # NOTE(sigmavirus24): Do not collect submodules, e.g.,
+            # if we have:
+            #  - flake8/
+            #  - flake8/plugins/
+            # Flake8 only needs ``flake8/`` to be provided. It will
+            # recurse on its own.
+            if package_directory.startswith(seen_package_directories):
+                continue
+
+            seen_package_directories += (package_directory,)
+            yield package_directory
+
+    def module_files(self):
+        """Collect the files listed as py_modules."""
+        modules = self.distribution.py_modules or []
+        filename_from = '{0}.py'.format
+        for module in modules:
+            yield filename_from(module)
+
+    def distribution_files(self):
+        """Collect package and module files."""
+        for package in self.package_files():
+            yield package
+
+        for module in self.module_files():
+            yield module
+
+        yield 'setup.py'
+
+    def run(self):
+        """Run the Flake8 application."""
+        flake8 = app.Application()
+        flake8.run(list(self.distribution_files()))
+        flake8.exit()

setup.py

@@ -76,8 +76,12 @@ setuptools.setup(
     ],
     install_requires=requires,
     entry_points={
-        'distutils.commands': ['flake8 = flake8.main:Flake8Command'],
-        'console_scripts': ['flake8 = flake8.main.cli:main'],
+        'distutils.commands': [
+            'flake8 = flake8.main.setuptools_command:Flake8'
+        ],
+        'console_scripts': [
+            'flake8 = flake8.main.cli:main'
+        ],
         'flake8.extension': [
             'F = flake8.plugins.pyflakes:FlakesChecker',
             # PEP-0008 checks provied by PyCQA/pycodestyle