Reorganised external code storage layout

imp/math/numbers/__init__.py

@@ -6,7 +6,7 @@ Terminology:
     the "prime proper divisors of n". 
 '''
 
-from imp.math.primefac import primefac
+from imp.extern.primefac import primefac
 
 def factors(n: int) -> int:
     pfactors: list[tuple[int, int]] = []

imp/math/numbers/functions.py

@@ -1,5 +1,3 @@
 def factorial(n: int) -> int:
     if n == 0: return 1
     return n * factorial(n-1)
-
-def