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/save.rs

@@ -0,0 +1,47 @@
+use nu_test_support::fs::{file_contents, Stub::FileWithContent};
+use nu_test_support::nu;
+use nu_test_support::playground::Playground;
+
+#[test]
+fn figures_out_intelligently_where_to_write_out_with_metadata() {
+    Playground::setup("save_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContent(
+            "cargo_sample.toml",
+            r#"
+                [package]
+                name = "nu"
+                version = "0.1.1"
+                authors = ["Yehuda Katz <wycats@gmail.com>"]
+                description = "A shell for the GitHub era"
+                license = "ISC"
+                edition = "2018"
+            "#,
+        )]);
+
+        let subject_file = dirs.test().join("cargo_sample.toml");
+
+        nu!(
+            cwd: dirs.root(),
+            "open save_test_1/cargo_sample.toml | save"
+        );
+
+        let actual = file_contents(&subject_file);
+        assert!(actual.contains("0.1.1"));
+    })
+}
+
+#[test]
+fn writes_out_csv() {
+    Playground::setup("save_test_2", |dirs, _| {
+        let expected_file = dirs.test().join("cargo_sample.csv");
+
+        nu!(
+            cwd: dirs.root(),
+            "open {}/cargo_sample.toml | get package | save save_test_2/cargo_sample.csv",
+            dirs.formats()
+        );
+
+        let actual = file_contents(expected_file);
+        assert!(actual.contains("[Table],A shell for the GitHub era,2018,ISC,nu,0.1.1"));
+    })
+}