Use _requirestxt logic for writing 'requires.txt'

This change extracts the logic for writing 'requires.txt' from
the ``egg_info`` module into ``_requirestxt``.

It also adds the missing pre-processing for ``install_requires`` and
``extras_require`` previously done in ``dist`` but now implemented in
``_requires.txt``.

setuptools/command/_requirestxt.py

@@ -6,11 +6,13 @@
 
 See https://setuptools.pypa.io/en/latest/deprecated/python_eggs.html#requires-txt
 """
+import io
 from collections import defaultdict
 from itertools import filterfalse
 from typing import Dict, List, Tuple, Mapping, TypeVar
 
 from .. import _reqs
+from ..extern.jaraco.text import yield_lines
 from ..extern.packaging.requirements import Requirement
 
 
@@ -94,3 +96,31 @@ def _clean_req(req):
 
 def _no_marker(req):
     return not req.marker
+
+
+def _write_requirements(stream, reqs):
+    lines = yield_lines(reqs or ())
+
+    def append_cr(line):
+        return line + '\n'
+    lines = map(append_cr, lines)
+    stream.writelines(lines)
+
+
+def write_requirements(cmd, basename, filename):
+    dist = cmd.distribution
+    data = io.StringIO()
+    install_requires, extras_require = _prepare(
+        dist.install_requires or (), dist.extras_require or {}
+    )
+    _write_requirements(data, install_requires)
+    for extra in sorted(extras_require):
+        data.write('\n[{extra}]\n'.format(**vars()))
+        _write_requirements(data, extras_require[extra])
+    cmd.write_or_delete_file("requirements", filename, data.getvalue())
+
+
+def write_setup_requirements(cmd, basename, filename):
+    data = io.StringIO()
+    _write_requirements(data, cmd.distribution.setup_requires)
+    cmd.write_or_delete_file("setup-requirements", filename, data.getvalue())

setuptools/command/egg_info.py

@@ -18,6 +18,7 @@
 
 from .._importlib import metadata
 from .. import _entry_points, _normalization
+from . import _requirestxt
 
 from setuptools import Command
 from setuptools.command.sdist import sdist
@@ -28,7 +29,6 @@
 from setuptools.glob import glob
 
 from setuptools.extern import packaging
-from setuptools.extern.jaraco.text import yield_lines
 from ..warnings import SetuptoolsDeprecationWarning
 
 
@@ -719,30 +719,9 @@ def warn_depends_obsolete(cmd, basename, filename):
         )
 
 
-def _write_requirements(stream, reqs):
-    lines = yield_lines(reqs or ())
-
-    def append_cr(line):
-        return line + '\n'
-    lines = map(append_cr, lines)
-    stream.writelines(lines)
-
-
-def write_requirements(cmd, basename, filename):
-    dist = cmd.distribution
-    data = io.StringIO()
-    _write_requirements(data, dist.install_requires)
-    extras_require = dist.extras_require or {}
-    for extra in sorted(extras_require):
-        data.write('\n[{extra}]\n'.format(**vars()))
-        _write_requirements(data, extras_require[extra])
-    cmd.write_or_delete_file("requirements", filename, data.getvalue())
-
-
-def write_setup_requirements(cmd, basename, filename):
-    data = io.StringIO()
-    _write_requirements(data, cmd.distribution.setup_requires)
-    cmd.write_or_delete_file("setup-requirements", filename, data.getvalue())
+# Export API used in entry_points
+write_requirements = _requirestxt.write_requirements
+write_setup_requirements = _requirestxt.write_setup_requirements
 
 
 def write_toplevel_names(cmd, basename, filename):