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/mod.rs

@@ -0,0 +1,35 @@
+mod append;
+mod calc;
+mod cd;
+mod compact;
+mod cp;
+mod default;
+mod edit;
+mod enter;
+mod first;
+mod format;
+mod get;
+mod group_by;
+mod histogram;
+mod insert;
+mod last;
+mod lines;
+mod ls;
+mod mkdir;
+mod mv;
+mod open;
+mod parse;
+mod pick;
+mod prepend;
+mod range;
+mod rename;
+mod reverse;
+mod rm;
+mod save;
+mod sort_by;
+mod split_by;
+mod split_column;
+mod touch;
+mod uniq;
+mod where_;
+mod wrap;