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/main.rs

@@ -0,0 +1,4 @@
+extern crate nu_test_support;
+
+mod commands;
+mod format_conversions;