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/calc.rs

@@ -0,0 +1,49 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn calculates_two_plus_two() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "2 + 2" | calc
+        "#
+    ));
+
+    assert!(actual.contains("4.0"));
+}
+
+#[test]
+fn calculates_two_to_the_power_six() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "2 ^ 6" | calc
+        "#
+    ));
+
+    assert!(actual.contains("64.0"));
+}
+
+#[test]
+fn calculates_three_multiplied_by_five() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "3 * 5" | calc
+        "#
+    ));
+
+    assert!(actual.contains("15.0"));
+}
+
+#[test]
+fn calculates_twenty_four_divided_by_two() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        echo "24 / 2" | calc
+        "#
+    ));
+
+    assert!(actual.contains("12.0"));
+}