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, 11 insertions, 0 deletions

diff --git a/setuptools/command/egg_info.py b/setuptools/command/egg_info.py
index 18b81340..ff009861 100644
--- a/setuptools/command/egg_info.py
+++ b/setuptools/command/egg_info.py
@@ -608,6 +608,17 @@ class manifest_maker(sdist):
         self.filelist.exclude_pattern(r'(^|' + sep + r')(RCS|CVS|\.svn)' + sep,
                                       is_regex=1)
 
+    def _safe_data_files(self, build_py):
+        """
+        The parent class implementation of this method (``sdist``) will try to
+        include data files, which might cause recursion problems, when
+        ``include_package_data=True``.
+
+        Therefore we have to avoid triggering any attempt of analyzing/building
+        the manifest again.
+        """
+        return build_py.get_data_files_without_manifest()
+
 
 def write_file(filename, contents):
     """Create a file with the specified name and write 'contents' (a