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/make/MakeSystem.cpp

@@ -8,8 +8,8 @@
 
 #include "goalc/make/Tools.h"
 
-#include "third-party/fmt/color.h"
-#include "third-party/fmt/core.h"
+#include "fmt/color.h"
+#include "fmt/core.h"
 
 std::string MakeStep::print() const {
   std::string result = fmt::format("Tool {} with inputs", tool);