Migrate from boost::filesystem to std::filesystem

[AlanGriffiths]

C++17 introduced std::filesystem, so it would be better to use that than Boost

But the default Ubuntu 20.04 standard library only has std::experimental::filesystem which is subtly different. So it may be worth deferring the migration until we move to base: core22

[AlanGriffiths]

#include <iostream>
#include <experimental/filesystem>
namespace fs = std::experimental::filesystem;
int main()
{
    for(fs::path p : {"/var/tmp/example.txt", "/", "/var/tmp/.", "foo", "./foo", "nonexistant/foo"})
        std::cout << "The parent path of " << p
                  << " is " << p.parent_path() 
		  << ", empty=" << p.parent_path().empty()
		  << ", exists=" << exists(p.parent_path()) << '\n';
}