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/default.rs

@@ -0,0 +1,36 @@
+use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn adds_row_data_if_column_missing() {
+    Playground::setup("default_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "los_tres_amigos.json",
+            r#"
+                {
+                    "amigos": [
+                        {"name":   "Yehuda"},
+                        {"name": "Jonathan", "rusty_luck": 0},
+                        {"name":   "Andres", "rusty_luck": 0},
+                        {"name":"GorbyPuff"}
+                    ]
+                }
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open los_tres_amigos.json
+                | get amigos
+                | default rusty_luck 1
+                | where rusty_luck == 1
+                | count
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "2");
+    });
+}