build: Move Git submodules to extern directory

* Move the https://gitlab.com/fastjet/fastjet.git and
  https://github.com/cms-externals/fastjet-contrib.git Git submodules
  into an 'extern' directory to make the directory management cleaner.

.gitmodules

@@ -1,9 +1,9 @@
 [submodule "fastjet"]
-	path = fastjet-core
+	path = extern/fastjet-core
 	url = https://gitlab.com/fastjet/fastjet.git
 [submodule "pybind11"]
 	path = pybind11
 	url = https://github.com/pybind/pybind11.git
 [submodule "fastjet-contrib"]
-	path = fastjet-contrib
+	path = extern/fastjet-contrib
 	url = https://github.com/cms-externals/fastjet-contrib.git

MANIFEST.in

@@ -1,7 +1,6 @@
 graft src
 graft tests
-graft fastjet-core
-graft fastjet-contrib
+graft extern
 graft pybind11
 global-exclude .git .gitmodules
 include LICENSE README.md pyproject.toml setup.py setup.cfg patch_clustersequence.txt

setup.py

@@ -24,9 +24,9 @@
 CGAL_ZIP = "https://github.com/CGAL/cgal/releases/download/v5.6/CGAL-5.6-library.zip"
 
 DIR = pathlib.Path(__file__).parent.resolve()
-FASTJET = DIR / "fastjet-core"
-FASTJET_CONTRIB = DIR / "fastjet-contrib"
-PYTHON = DIR / "src/fastjet"
+FASTJET = DIR / "extern" / "fastjet-core"
+FASTJET_CONTRIB = DIR / "extern" / "fastjet-contrib"
+PYTHON = DIR / "src" / "fastjet"
 OUTPUT = PYTHON / "_fastjet_core"