Fix #7095: dirhtml: Cross references are broken via intersphinx and :doc:

tests/roots/test-builder-dirhtml/bar.rst

@@ -0,0 +1,4 @@
+.. _bar:
+
+bar
+===

tests/roots/test-builder-dirhtml/foo/foo_1.rst

@@ -0,0 +1,4 @@
+.. _foo_1:
+
+foo/foo_1
+=========

tests/roots/test-builder-dirhtml/foo/foo_2.rst

@@ -0,0 +1,4 @@
+.. _foo_2:
+
+foo/foo_2
+=========

tests/roots/test-builder-dirhtml/foo/index.rst

@@ -0,0 +1,9 @@
+.. _foo:
+
+foo/index
+=========
+
+.. toctree::
+
+   foo_1
+   foo_2

tests/roots/test-builder-dirhtml/index.rst

@@ -0,0 +1,9 @@
+.. _index:
+
+index
+=====
+
+.. toctree::
+
+   foo/index
+   bar