Obey precedence rules in which; Fix #2875 (#2885)

* 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

crates/nu-cli/tests/commands/mod.rs

@@ -56,5 +56,6 @@ mod touch;
 mod uniq;
 mod update;
 mod where_;
+mod which;
 mod with_env;
 mod wrap;

crates/nu-cli/tests/commands/which.rs

@@ -0,0 +1,96 @@
+use nu_test_support::nu;
+
+#[test]
+fn which_ls() {
+    let actual = nu!(
+        cwd: ".",
+        "which ls | get path | str trim"
+    );
+
+    assert_eq!(actual.out, "Nushell built-in command");
+}
+
+#[test]
+fn which_alias_ls() {
+    let actual = nu!(
+        cwd: ".",
+        "alias ls = ls -a; which ls | get path | str trim"
+    );
+
+    assert_eq!(actual.out, "Nushell alias");
+}
+
+#[test]
+fn which_def_ls() {
+    let actual = nu!(
+        cwd: ".",
+        "def ls [] {echo def}; which ls | get path | str trim"
+    );
+
+    assert_eq!(actual.out, "Nushell custom command");
+}
+
+#[test]
+fn correct_precedence_alias_def_custom() {
+    let actual = nu!(
+        cwd: ".",
+        "def ls [] {echo def}; alias ls = echo alias; which ls | get path | str trim"
+    );
+
+    assert_eq!(actual.out, "Nushell alias");
+}
+
+#[test]
+fn multiple_reports_for_alias_def_custom() {
+    let actual = nu!(
+        cwd: ".",
+        "def ls [] {echo def}; alias ls = echo alias; which -a ls | count"
+    );
+
+    let count: i32 = actual.out.parse().unwrap();
+    assert!(count >= 3);
+}
+
+// `get_aliases_with_name` and `get_custom_commands_with_name` don't return the correct count of
+// values
+// I suspect this is due to the ScopeFrame getting discarded at '}' and the command is then
+// executed in the parent scope
+// See: parse_definition, line 2187 for reference.
+#[ignore]
+#[test]
+fn multiple_reports_of_multiple_alias() {
+    let actual = nu!(
+        cwd: ".",
+        "alias xaz = echo alias1; def helper [] {alias xaz = echo alias2; which -a xaz}; helper | count"
+    );
+
+    let count: i32 = actual.out.parse().unwrap();
+    assert!(count == 2);
+}
+
+#[ignore]
+#[test]
+fn multiple_reports_of_multiple_defs() {
+    let actual = nu!(
+        cwd: ".",
+        "def xaz [] {echo def1}; def helper [] { def xaz [] { echo def2 }; which -a xaz }; helper | count"
+    );
+
+    let count: i32 = actual.out.parse().unwrap();
+    assert!(count == 2);
+}
+
+//Fails due to ParserScope::add_definition
+// frame.custom_commands.insert(name.clone(), block.clone());
+// frame.commands.insert(name, whole_stream_command(block));
+#[ignore]
+#[test]
+fn def_only_seen_once() {
+    let actual = nu!(
+        cwd: ".",
+        "def xaz [] {echo def1}; which -a xaz | count"
+    );
+    //count is 2. One custom_command (def) one built in ("wrongly" added)
+    let count: i32 = actual.out.parse().unwrap();
+    assert!(count == 1);
+}