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/split_by.rs

@@ -0,0 +1,55 @@
+use nu_test_support::fs::Stub::{EmptyFile, FileWithContentToBeTrimmed};
+use nu_test_support::playground::Playground;
+use nu_test_support::{nu, nu_error, pipeline};
+
+#[test]
+fn splits() {
+    Playground::setup("split_by_test_1", |dirs, sandbox| {
+        sandbox.with_files(vec![FileWithContentToBeTrimmed(
+            "los_tres_caballeros.csv",
+            r#"
+                first_name,last_name,rusty_at,type
+                Andrés,Robalino,10/11/2013,A
+                Jonathan,Turner,10/12/2013,B
+                Yehuda,Katz,10/11/2013,A
+            "#,
+        )]);
+
+        let actual = nu!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                open los_tres_caballeros.csv
+                | group-by rusty_at
+                | split-by type
+                | get A."10/11/2013"
+                | count
+                | echo $it
+            "#
+        ));
+
+        assert_eq!(actual, "2");
+    })
+}
+
+#[test]
+fn errors_if_no_table_given_as_input() {
+    Playground::setup("split_by_test_2", |dirs, sandbox| {
+        sandbox.with_files(vec![
+            EmptyFile("los.txt"),
+            EmptyFile("tres.txt"),
+            EmptyFile("amigos.txt"),
+            EmptyFile("arepas.clu"),
+        ]);
+
+        let actual = nu_error!(
+            cwd: dirs.test(), pipeline(
+            r#"
+                ls
+                | get name
+                | split-by type
+            "#
+        ));
+
+        assert!(actual.contains("Expected table from pipeline"));
+    })
+}