fix: preserve package's source when it has extras (#6472)

This PR splits out simple part of #6130. Once this merged I'll update
that PR to contain only second (more controversial) fix.

src/poetry/puzzle/provider.py

@@ -603,7 +603,15 @@ class Provider:
             )
             )
             package = dependency_package.package
             package = dependency_package.package
             dependency = dependency_package.dependency
             dependency = dependency_package.dependency
-            _dependencies.append(package.without_features().to_dependency())
+            new_dependency = package.without_features().to_dependency()
+
+            # When adding dependency foo[extra] -> foo, preserve foo's source, if it's
+            # specified. This prevents us from trying to get foo from PyPI
+            # when user explicitly set repo for foo[extra].
+            if not new_dependency.source_name and dependency.source_name:
+                new_dependency.source_name = dependency.source_name
+
+            _dependencies.append(new_dependency)
 
 
         for dep in requires:
         for dep in requires:
             if not self._python_constraint.allows_any(dep.python_constraint):
             if not self._python_constraint.allows_any(dep.python_constraint):

tests/puzzle/test_provider.py

@@ -688,3 +688,31 @@ def test_complete_package_preserves_source_type_with_subdirectories(
         dependency_one_copy.to_pep_508(),
         dependency_one_copy.to_pep_508(),
         dependency_two.to_pep_508(),
         dependency_two.to_pep_508(),
     }
     }
+
+
+@pytest.mark.parametrize("source_name", [None, "repo"])
+def test_complete_package_with_extras_preserves_source_name(
+    provider: Provider, repository: Repository, source_name: str | None
+) -> None:
+    package_a = Package("A", "1.0")
+    package_b = Package("B", "1.0")
+    dep = get_dependency("B", "^1.0", optional=True)
+    package_a.add_dependency(dep)
+    package_a.extras = {"foo": [dep]}
+    repository.add_package(package_a)
+    repository.add_package(package_b)
+
+    dependency = Dependency("A", "1.0", extras=["foo"])
+    if source_name:
+        dependency.source_name = source_name
+
+    complete_package = provider.complete_package(
+        DependencyPackage(dependency, package_a)
+    )
+
+    requires = complete_package.package.all_requires
+    assert len(requires) == 2
+    assert requires[0].name == "a"
+    assert requires[0].source_name == source_name
+    assert requires[1].name == "b"
+    assert requires[1].source_name is None