Remove comparator.hh and switch to <=> in a bunch of places

Known behavior changes:

- `MemorySourceAccessor`'s comparison operators no longer forget to
  compare the `SourceAccessor` base class.

Progress on #10832

What remains for that issue is hopefully much easier!

src/libstore/path-info.cc

@@ -3,11 +3,13 @@
 #include "path-info.hh"
 #include "store-api.hh"
 #include "json-utils.hh"
+#include "comparator.hh"
 
 namespace nix {
 
 GENERATE_CMP_EXT(
     ,
+    std::weak_ordering,
     UnkeyedValidPathInfo,
     me->deriver,
     me->narHash,
@@ -19,12 +21,6 @@ GENERATE_CMP_EXT(
     me->sigs,
     me->ca);
 
-GENERATE_CMP_EXT(
-    ,
-    ValidPathInfo,
-    me->path,
-    static_cast<const UnkeyedValidPathInfo &>(*me));
-
 std::string ValidPathInfo::fingerprint(const Store & store) const
 {
     if (narSize == 0)