deps: update fmt to latest version (#3403)

This updates `fmt` to the latest version and moves to just being a copy
of their repo to make updating easier (no editing their cmake / figuring
out which files to minimally include).

The motivation for this is now that we switched to C++ 20, there were a
ton of deprecated function usages that is going away in future compiler
versions. This gets rid of all those warnings.

goalc/debugger/DebugInfo.cpp

@@ -2,7 +2,7 @@
 
 #include <utility>
 
-#include "third-party/fmt/core.h"
+#include "fmt/core.h"
 
 DebugInfo::DebugInfo(std::string obj_name) : m_obj_name(std::move(obj_name)) {}
 
@@ -37,4 +37,4 @@ std::string DebugInfo::disassemble_function_by_name(const std::string& name,
     }
   }
   return result;
-}
+}