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/format_conversions/sqlite.rs

@@ -0,0 +1,19 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn table_to_sqlite_and_back_into_table() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            open sample.db
+            | to-sqlite
+            | from-sqlite
+            | get table_values
+            | nth 2
+            | get x
+            | echo $it
+        "#
+    ));
+
+    assert_eq!(actual, "hello");
+}