Don't suppress ignore for specific case of unique packages in nix

Nix has changed snapshot scheme from YYYY-MM-DD to 0-unstable-YYYY-MM-DD,
so when ignored statuses are suppressed for projects which are only
packaged in nix, older version outdate newever versions because of
changed schema. Disallow ignored suppression in that specific case.

Fixes repology/repology-rules#823

repology/classifier/__init__.py

@@ -145,6 +145,23 @@ def _is_packageset_unique(packages: Sequence[Package]) -> bool:
     return True
 
 
+def _is_nix_mixed_snapshot_schemes_case(packages: Sequence[Package]) -> bool:
+    # hack/special case: nix has changed snapshot scheme from YYYY-MM-DD to
+    # 0-unstable-YYYY-MM-DD since the former compares as greater, we can't
+    # suppress ignore for nix, as it would make older snapshots outdated
+    # newer ones
+    has_old_scheme = False
+    has_new_scheme = False
+    if packages[0].family == 'nix':
+        for package in packages:
+            if package.version.startswith('20'):
+                has_old_scheme = True
+            elif package.version.startswith('0-unstable-'):
+                has_new_scheme = True
+
+    return has_old_scheme and has_new_scheme
+
+
 def _should_suppress_ignore(packages: Sequence[Package]) -> bool:
     if len(packages) <= 1:
         return True
@@ -160,6 +177,9 @@ def _should_suppress_ignore(packages: Sequence[Package]) -> bool:
         if package.has_flag(PackageFlags.NOSCHEME):
             return False
 
+    if _is_nix_mixed_snapshot_schemes_case(packages):
+        return False
+
     return True