Fix 1461: Better loop breaker for `manifest_maker`

The inconsistency for the `package_data` configuration in sdists
when `include_package_data=True` in #1461 have been causing some
problems for the community for a while, as also shown in #2835.

As pointed out by
[@jaraco](https://github.com/pypa/setuptools/issues/1461#issuecomment-749092366),
this was being caused by a mechanism to break the recursion between the
`egg_info` and `sdist` commands.

In summary the loop is caused by the following behaviour:

- the `egg_info` command uses a subclass of `sdist` (`manifest_maker`)
  to calculate the MANIFEST,
- the `sdist` class needs to know the MANIFEST to calculate the data files when
  `include_package_data=True`

Previously, the mechanism to break this loop was to simply ignore
the data files in `sdist` when `include_package_data=True`.

The approach implemented in this change was to replace this mechanism,
by allowing `manifest_maker` to override the `_safe_data_files` method
from `sdist`.

---

Please notice [an extensive experiment]
(https://github.com/abravalheri/experiment-setuptools-package-data)
was carried out to investigate the previous confusing behaviour.

There is also [a simplified theoretical analysis]
(https://github.com/pyscaffold/pyscaffold/pull/535#issuecomment-956296407)
comparing the observed behavior in the experiment and the expected
one. This analysis point out to the same offender indicated by
[@jaraco](https://github.com/pypa/setuptools/issues/1461#issuecomment-749092366)
(which is being replaced in this change).

1 files changed, 8 insertions, 5 deletions

diff --git a/setuptools/command/sdist.py b/setuptools/command/sdist.py
index e8062f2e..0285b690 100644
--- a/setuptools/command/sdist.py
+++ b/setuptools/command/sdist.py
@@ -114,12 +114,15 @@ class sdist(sdist_add_defaults, orig.sdist):
 
     def _safe_data_files(self, build_py):
         """
-        Extracting data_files from build_py is known to cause
-        infinite recursion errors when `include_package_data`
-        is enabled, so suppress it in that case.
+        Since the ``sdist`` class is also used to compute the MANIFEST
+        (via :obj:`setuptools.command.egg_info.manifest_maker`),
+        there might be recursion problems when trying to obtain the list of
+        data_files and ``include_package_data=True`` (which in turn depends on
+        the files included in the MANIFEST).
+
+        To avoid that, ``manifest_maker`` should be able to overwrite this
+        method and avoid recursive attempts to build/analyze the MANIFEST.
         """
-        if self.distribution.include_package_data:
-            return ()
         return build_py.data_files
 
     def _add_data_files(self, data_files):