Add test case for manual stub files

Author: brandonchinn178

examples/demo/manual_stubs/BUILD.bazel

@@ -0,0 +1,13 @@
+load("@rules_python//python:py_library.bzl", "py_library")
+
+py_library(
+    name = "lib",
+    srcs = glob(["lib/**/*.py"]),
+    deps = [":foo"],
+)
+
+py_library(
+    name = "foo",
+    imports = ["."],
+    pyi_srcs = glob(["foo/**/*.pyi"]),
+)

examples/demo/manual_stubs/foo/__init__.pyi

@@ -0,0 +1,2 @@
+class Foo:
+    pass

examples/demo/manual_stubs/lib/main.py

@@ -0,0 +1 @@
+from foo import Foo