Include submodules in sdists and github actions

.github/workflows/python-package.yml

@@ -14,6 +14,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           fetch-depth: 0
+          submodules: recursive
 
       - name: Set up Python
         uses: actions/setup-python@v3.1.2
@@ -57,6 +58,7 @@ jobs:
       - uses: actions/checkout@v3
         with:
           fetch-depth: 0
+          submodules: recursive
 
       - name: Install libomp
         if: runner.os == 'macOS'

MANIFEST.in

@@ -0,0 +1 @@
+graft extern