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/src/shell.rs

@@ -0,0 +1,11 @@
+#![allow(clippy::module_inception)]
+
+pub(crate) mod completer;
+pub(crate) mod filesystem_shell;
+pub(crate) mod help_shell;
+pub(crate) mod helper;
+pub(crate) mod shell;
+pub(crate) mod shell_manager;
+pub(crate) mod value_shell;
+
+pub(crate) use helper::Helper;