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/mod.rs

@@ -0,0 +1,11 @@
+mod bson;
+mod csv;
+mod json;
+mod ods;
+mod sqlite;
+mod ssv;
+mod toml;
+mod tsv;
+mod url;
+mod xlsx;
+mod yaml;