* Obay precedence rules in which; Fix#2875
Before which did not obay the precedence of alias before def commands.
Furthermore, `which -a echo` would only report either an alias or a def command or an
internal command with the provided name. Not all.
With this commit applied its fixed :)
Example:
```shell
/home/leo/repos/nushell(fix/which_reports_wrong_usage)> def echo [] {^echo hi}
/home/leo/repos/nushell(fix/which_reports_wrong_usage)> echo
hi
/home/leo/repos/nushell(fix/which_reports_wrong_usage)> which -a echo
───┬──────┬──────────────────────────┬─────────
# │ arg │ path │ builtin
───┼──────┼──────────────────────────┼─────────
0 │ echo │ Nushell custom command │ No
1 │ echo │ Nushell built-in command │ Yes
2 │ echo │ /usr/bin/echo │ No
───┴──────┴──────────────────────────┴─────────
/home/leo/repos/nushell(fix/which_reports_wrong_usage)> alias echo = ^echo hi there
/home/leo/repos/nushell(fix/which_reports_wrong_usage)> echo
hi there
/home/leo/repos/nushell(fix/which_reports_wrong_usage)> which -a echo
───┬──────┬──────────────────────────┬─────────
# │ arg │ path │ builtin
───┼──────┼──────────────────────────┼─────────
0 │ echo │ Nushell alias │ No
1 │ echo │ Nushell custom command │ No
2 │ echo │ Nushell built-in command │ Yes
3 │ echo │ /usr/bin/echo │ No
───┴──────┴──────────────────────────┴─────────
```
* Fix clippy lint
* Fix vec always Some even if empty
