refactor: give common.py's parts their own modules

common.py had grown to 1833 lines by accumulation. Six coherent pieces
move out - untrusted parsing, digests, archives, generated artefacts,
release assets, dump catalogues - and common.py re-exports them, so the
sixty existing import sites keep working and migrating them stays
optional.

The site build is now reproducible, which is what made the move
checkable. It deleted its generated directories first, so every page was
new and write_if_changed had no earlier version to compare against: a
deploy republished six hundred pages for the clock alone. Directories
are swept instead, a page is removed only once nothing produces it, and
the body pass compares against the body of the file on disk rather than
against the decorated page. Two consecutive builds on the same inputs
now produce identical bytes; before, 1034 files differed.

tests/test_e2e.py

@@ -28,6 +28,7 @@ from pathlib import Path
 
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), "..", "scripts"))
 
+import largefiles
 import yaml
 from common import (
     build_zip_contents_index,
@@ -5120,8 +5121,8 @@ struct BurnDriver BurnDrvneogeo = {
                 raise urllib.error.HTTPError(url, 404, "Not Found", {}, None)
             return FakeResponse(b"payload")
 
-        original = common.urllib.request.urlopen
-        common.urllib.request.urlopen = fake_urlopen
+        original = largefiles.urllib.request.urlopen
+        largefiles.urllib.request.urlopen = fake_urlopen
         try:
             with tempfile.TemporaryDirectory() as tmpdir:
                 path = common.fetch_large_file(
@@ -5131,7 +5132,7 @@ struct BurnDriver BurnDrvneogeo = {
                 with open(path, "rb") as fh:
                     self.assertEqual(fh.read(), b"payload")
         finally:
-            common.urllib.request.urlopen = original
+            largefiles.urllib.request.urlopen = original
 
         self.assertEqual(len(attempts), 2, attempts)
         self.assertIn("MAME%200.174", attempts[0])