locker: refactor dependency walk logic

Resolves: #5141

src/poetry/utils/exporter.py

 from __future__ import annotations
 
-import itertools
 import urllib.parse
 
 from typing import TYPE_CHECKING
@@ -70,21 +69,22 @@ class Exporter:
         content = ""
         dependency_lines = set()
 
-        for package, groups in itertools.groupby(
-            self._poetry.locker.get_project_dependency_packages(
-                project_requires=self._poetry.package.all_requires,
-                dev=dev,
-                extras=extras,
-            ),
-            lambda dependency_package: dependency_package.package,
+        # Get project dependencies.
+        root_package = (
+            self._poetry.package.clone()
+            if dev
+            else self._poetry.package.with_dependency_groups(["default"], only=True)
+        )
+
+        for dependency_package in self._poetry.locker.get_project_dependency_packages(
+            project_requires=root_package.all_requires,
+            project_python_marker=root_package.python_marker,
+            dev=dev,
+            extras=extras,
         ):
             line = ""
-            dependency_packages = list(groups)
-            dependency = dependency_packages[0].dependency
-            marker = dependency.marker
-            for dep_package in dependency_packages[1:]:
-                marker = marker.union(dep_package.dependency.marker)
-            dependency.marker = marker
+            dependency = dependency_package.dependency
+            package = dependency_package.package
 
             if package.develop:
                 line += "-e "

tests/console/commands/test_export.py

@@ -84,7 +84,9 @@ def _export_requirements(tester: CommandTester, poetry: Poetry) -> None:
     assert poetry.locker.lock.exists()
 
     expected = """\
-foo==1.0.0
+foo==1.0.0 ;\
+ python_version >= "2.7" and python_version < "2.8" or\
+ python_version >= "3.4" and python_version < "4.0"
 """
 
     assert content == expected
@@ -113,7 +115,9 @@ def test_export_fails_on_invalid_format(tester: CommandTester, do_lock: None):
 def test_export_prints_to_stdout_by_default(tester: CommandTester, do_lock: None):
     tester.execute("--format requirements.txt")
     expected = """\
-foo==1.0.0
+foo==1.0.0 ;\
+ python_version >= "2.7" and python_version < "2.8" or\
+ python_version >= "3.4" and python_version < "4.0"
 """
     assert tester.io.fetch_output() == expected
 
@@ -123,7 +127,9 @@ def test_export_uses_requirements_txt_format_by_default(
 ):
     tester.execute()
     expected = """\
-foo==1.0.0
+foo==1.0.0 ;\
+ python_version >= "2.7" and python_version < "2.8" or\
+ python_version >= "3.4" and python_version < "4.0"
 """
     assert tester.io.fetch_output() == expected
 
@@ -131,8 +137,12 @@ foo==1.0.0
 def test_export_includes_extras_by_flag(tester: CommandTester, do_lock: None):
     tester.execute("--format requirements.txt --extras feature_bar")
     expected = """\
-bar==1.1.0
-foo==1.0.0
+bar==1.1.0 ;\
+ python_version >= "2.7" and python_version < "2.8" or\
+ python_version >= "3.4" and python_version < "4.0"
+foo==1.0.0 ;\
+ python_version >= "2.7" and python_version < "2.8" or\
+ python_version >= "3.4" and python_version < "4.0"
 """
     assert tester.io.fetch_output() == expected