Move most of the root package into a subcrate. (#1445)

This improves incremental build time when working on what was previously
the root package. For example, previously all plugins would be rebuilt
with a change to `src/commands/classified/external.rs`, but now only
`nu-cli` will have to be rebuilt (and anything that depends on it).

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

@@ -0,0 +1,86 @@
+use nu_test_support::fs::{files_exist_at, Stub::EmptyFile};
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, nu_error};
+use std::path::Path;
+
+#[test]
+fn knows_the_filesystems_entered() {
+    Playground::setup("enter_test_1", |dirs, sandbox| {
+        sandbox
+            .within("red_pill")
+            .with_files(vec![
+                EmptyFile("andres.nu"),
+                EmptyFile("jonathan.nu"),
+                EmptyFile("yehuda.nu"),
+            ])
+            .within("blue_pill")
+            .with_files(vec![
+                EmptyFile("bash.nxt"),
+                EmptyFile("korn.nxt"),
+                EmptyFile("powedsh.nxt"),
+            ])
+            .mkdir("expected");
+
+        let red_pill_dir = dirs.test().join("red_pill");
+        let blue_pill_dir = dirs.test().join("blue_pill");
+        let expected = dirs.test().join("expected");
+        let expected_recycled = expected.join("recycled");
+
+        nu!(
+            cwd: dirs.test(),
+            r#"
+                enter expected
+                mkdir recycled
+                enter ../red_pill
+                mv jonathan.nu ../expected
+                enter ../blue_pill
+                cp *.nxt ../expected/recycled
+                p
+                p
+                mv ../red_pill/yehuda.nu .
+                n
+                mv andres.nu ../expected/andres.nu
+                exit
+                cd ..
+                rm red_pill --recursive
+                exit
+                n
+                rm blue_pill --recursive
+                exit
+            "#
+        );
+
+        assert!(!red_pill_dir.exists());
+        assert!(files_exist_at(
+            vec![
+                Path::new("andres.nu"),
+                Path::new("jonathan.nu"),
+                Path::new("yehuda.nu"),
+            ],
+            expected
+        ));
+
+        assert!(!blue_pill_dir.exists());
+        assert!(files_exist_at(
+            vec![
+                Path::new("bash.nxt"),
+                Path::new("korn.nxt"),
+                Path::new("powedsh.nxt"),
+            ],
+            expected_recycled
+        ));
+    })
+}
+
+#[test]
+fn errors_if_file_not_found() {
+    Playground::setup("enter_test_2", |dirs, _| {
+        let actual = nu_error!(
+            cwd: dirs.test(),
+            "enter i_dont_exist.csv"
+        );
+
+        assert!(actual.contains("File could not be opened"));
+        assert!(actual.contains("file not found"));
+    })
+}