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/touch.rs

@@ -0,0 +1,16 @@
+use nu_test_support::nu;
+use nu_test_support::playground::Playground;
+
+#[test]
+fn creates_a_file_when_it_doesnt_exist() {
+    Playground::setup("create_test_1", |dirs, _sandbox| {
+        nu!(
+            cwd: dirs.test(),
+            "touch i_will_be_created.txt"
+        );
+
+        let path = dirs.test().join("i_will_be_created.txt");
+
+        assert!(path.exists());
+    })
+}