sphinxdocs: make bazel package xrefs work (#2903)

This allows using xrefs like `//python/runtime_env_toolchains` and
`runtime_env_toolchains`.

sphinxdocs/src/sphinx_bzl/bzl.py

diff --git a/sphinxdocs/src/sphinx_bzl/bzl.py b/sphinxdocs/src/sphinx_bzl/bzl.py
index 7804e7f..4468ec6 100644
--- a/sphinxdocs/src/sphinx_bzl/bzl.py
+++ b/sphinxdocs/src/sphinx_bzl/bzl.py
@@ -394,7 +394,7 @@
                 # This allows referencing an arg as e.g `funcname.argname`
                 anchor_id,
                 # This allows referencing an arg as simply `argname`
-                arg_name
+                arg_name,
             ],
         )
 
@@ -503,7 +503,22 @@
         self.env.ref_context["bzl:object_id_stack"] = []
         self.env.ref_context["bzl:doc_id_stack"] = []
 
-        _, _, basename = file_label.partition(":")
+        package_label, _, basename = file_label.partition(":")
+
+        # Transform //foo/bar:BUILD.bazel into "bar"
+        # This allows referencing "bar" as itself
+        extra_alt_names = []
+        if basename in ("BUILD.bazel", "BUILD"):
+            # Allow xref //foo
+            extra_alt_names.append(package_label)
+            basename = os.path.basename(package_label)
+            # Handle //:BUILD.bazel
+            if not basename:
+                # There isn't a convention for referring to the root package
+                # besides `//:`, which is already the file_label. So just
+                # use some obvious value
+                basename = "__ROOT_BAZEL_PACKAGE__"
+
         index_description = f"File {label}"
         absolute_label = repo + label
         self.env.get_domain("bzl").add_object(
@@ -527,7 +542,8 @@
                 file_label,
                 # Allow xref bar.bzl
                 basename,
-            ],
+            ]
+            + extra_alt_names,
         )
         index_node = addnodes.index(
             entries=[

sphinxdocs/tests/sphinx_stardoc/sphinx_output_test.py

diff --git a/sphinxdocs/tests/sphinx_stardoc/sphinx_output_test.py b/sphinxdocs/tests/sphinx_stardoc/sphinx_output_test.py
index aa21369..c78089a 100644
--- a/sphinxdocs/tests/sphinx_stardoc/sphinx_output_test.py
+++ b/sphinxdocs/tests/sphinx_stardoc/sphinx_output_test.py
@@ -67,6 +67,8 @@
         ("tag_class_attr_using_attr_role_just_attr_name", "ta1", "module_extension.html#myext.mytag.ta1"),
         ("file_without_repo", "//lang:rule.bzl", "rule.html"),
         ("file_with_repo", "@testrepo//lang:rule.bzl", "rule.html"),
+        ("package_absolute", "//lang", "target.html"),
+        ("package_basename", "lang", "target.html"),
         # fmt: on
     )
     def test_xrefs(self, text, href):

sphinxdocs/tests/sphinx_stardoc/xrefs.md

diff --git a/sphinxdocs/tests/sphinx_stardoc/xrefs.md b/sphinxdocs/tests/sphinx_stardoc/xrefs.md
index a32bb10..bbd415c 100644
--- a/sphinxdocs/tests/sphinx_stardoc/xrefs.md
+++ b/sphinxdocs/tests/sphinx_stardoc/xrefs.md
@@ -51,3 +51,8 @@
 
 * without repo {obj}`//lang:rule.bzl`
 * with repo {obj}`@testrepo//lang:rule.bzl`
+
+## Package refs
+
+* absolute label {obj}`//lang`
+* package basename {obj}`lang`