[requirements] organize multi-level requirements in extra directory

requirements/base.in

@@ -0,0 +1 @@
+django>=5.1,<5.2

requirements/base.txt

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+#
+# This file is autogenerated by pip-compile with Python 3.13
+# by the following command:
+#
+#    pip-compile requirements/base.in
+#
+asgiref==3.8.1
+    # via django
+django==5.1.3
+    # via -r requirements/base.in
+sqlparse==0.5.2
+    # via django