Allow custom-command to "install" itself

The comment in pypa/setuptools#2591 (comment)
reports that users wanting to use the custom-command plugin need to
explicitly add it using `setup.py`.

This commit tried to demonstrate how that can be done without requiring
the user to change their `setup.py`.

The "installation" of the new sub-command is done via the
`finalize_distribution_options` hook.

custom/build.py

@@ -11,3 +11,9 @@ def finalize_options(self):
 
     def run(self):
         log.info("brrrrrrrrr")
+
+
+def install(dist):
+    # This could be protected by a opt-in condition...
+    build = dist.get_command_obj("build")
+    build.sub_commands = [*build.sub_commands, ("custom-build", None)]

setup.cfg

@@ -7,3 +7,5 @@ packages = find_namespace:
 [options.entry_points]
 distutils.commands =
 	custom-build = custom.build:Command
+setuptools.finalize_distribution_options =
+	custom-build = custom.build:install