Allow modules to use other modules (#6162)

* Allow private imports inside modules

Can call `use ...` inside modules now.

* Add more tests

* Add a leak test

* Refactor exportables; Prepare for 'export use'

* Fix description

* Implement 'export use' command

This allows re-exporting module's commands and aliases from another
module.

* Add more tests; Fix import pattern list strings

The import pattern strings didn't trim the surrounding quotes.

* Add ignored test

crates/nu-protocol/src/exportable.rs

@@ -1,7 +1,7 @@
 use crate::{AliasId, BlockId, DeclId};
 
 pub enum Exportable {
-    Decl(DeclId),
-    Alias(AliasId),
-    EnvVar(BlockId),
+    Decl { name: Vec<u8>, id: DeclId },
+    Alias { name: Vec<u8>, id: AliasId },
+    EnvVar { name: Vec<u8>, id: BlockId },
 }