Rename setuptools.config.{setupcfg => legacy_setupcfg}

In pypa#2685 the plan that the community seems to agree on is to always
automatically translate `setup.cfg` into a `pyproject.toml` equivalent
and then proceed reading/parsing the configuration.

The objective of this change is to open space so we can implement this
way of reading `setup.cfg` in a new `setuptools.config.setupcfg` module
while keeping the legacy way of handling `setup.cfg` around.

The rationale behind keeping the legacy way around is that, to avoid
breaking existing packages during a transition period, we can compare
the old and the new way of parsing the configuration (e.g. via
`setuptools.{metadata,options}.compare`) and in the case they are
conflicting, use the old way and emit a warning asking the user to
report the error.

setuptools/config/__init__.py

@@ -1,6 +1,6 @@
 # For backward compatibility, the following classes/functions are exposed
 # from `config.setupcfg`
-from setuptools.config.setupcfg import (
+from setuptools.config.legacy_setupcfg import (
     ConfigHandler,
     parse_configuration,
     read_configuration,