Allow expanding aliases before keywords, improve hiding (#4858)

* Allow aliasing source

* Add test

* improve hiding

* Finish adding tests

* fix test

crates/nu-command/tests/commands/alias.rs

@@ -0,0 +1,40 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn alias_simple() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        alias bar = source sample_def.nu; bar; greet
+        "#
+    ));
+
+    assert_eq!(actual.out, "hello");
+}
+
+#[test]
+fn alias_hiding1() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        source ./activate-foo.nu;
+        $nu.scope.aliases | find deactivate-foo | length
+        "#
+    ));
+
+    assert_eq!(actual.out, "1");
+}
+
+#[test]
+fn alias_hiding2() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+        source ./activate-foo.nu;
+        deactivate-foo;
+        $nu.scope.aliases | find deactivate-foo | length
+        "#
+    ));
+
+    assert_eq!(actual.out, "0");
+}