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/libutil/source-accessor.hh

@@ -152,9 +152,9 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
         return number == x.number;
     }
 
-    bool operator < (const SourceAccessor & x) const
+    auto operator <=> (const SourceAccessor & x) const
     {
-        return number < x.number;
+        return number <=> x.number;
     }
 
     void setPathDisplay(std::string displayPrefix, std::string displaySuffix = "");