Use std::filesystem functions in more places

This makes for shorter and more portable code.

The only tricky part is catching exceptions: I just searched for near by
`catch (Error &)` or `catch (SysError &)` and adjusted them to `catch
(std::filesystem::filesystem_error &)` according to my human judgement.

Good for windows portability; will help @siddhantk232 with his GSOC
project.

src/libcmd/repl.cc

@@ -264,6 +264,7 @@ StringSet NixRepl::completePrefix(const std::string & prefix)
                     completions.insert(prev + dir + "/" + entry.name);
             }
         } catch (Error &) {
+        } catch (std::filesystem::filesystem_error &) {
         }
     } else if ((dot = cur.rfind('.')) == std::string::npos) {
         /* This is a variable name; look it up in the current scope. */