related to nushell#9773

…to-standard-library-ci

this commit allows to define constants in modules and test this PR
in the REPL.

related to nushell#9773

<!--
if this PR closes one or more issues, you can automatically link the PR
with
them by using one of the [*linking
keywords*](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword),
e.g.
- this PR should close #xxxx
- fixes #xxxx

you can also mention related issues, PRs or discussions!
-->

# Description
<!--
Thank you for improving Nushell. Please, check our [contributing
guide](../CONTRIBUTING.md) and talk to the core team before making major
changes.

Description of your pull request goes here. **Provide examples and/or
screenshots** if your changes affect the user experience.
-->

#9773 introduced constants to
modules and allowed to export them, but only within one level. This PR:
* allows recursive exporting of constants from all submodules
* fixes submodule imports in a list import pattern
* makes sure exported constants are actual constants

Should unblock #9678

### Example:
```nushell
module spam {
    export module eggs {
        export module bacon {
            export const viking = 'eats'
        }
    }
}

use spam 
print $spam.eggs.bacon.viking  # prints 'eats'

use spam [eggs]
print $eggs.bacon.viking  # prints 'eats'

use spam eggs bacon viking
print $viking  # prints 'eats'
```

### Limitation 1:

Considering the above `spam` module, attempting to get `eggs bacon` from
`spam` module doesn't work directly:
```nushell
use spam [ eggs bacon ]  # attempts to load `eggs`, then `bacon`
use spam [ "eggs bacon" ]  # obviously wrong name for a constant, but doesn't work also for commands
```

Workaround (for example):
```nushell
use spam eggs
use eggs [ bacon ]

print $bacon.viking  # prints 'eats'
```

I'm thinking I'll just leave it in, as you can easily work around this.
It is also a limitation of the import pattern in general, not just
constants.

### Limitation 2:

`overlay use` successfully imports the constants, but `overlay hide`
does not hide them, even though it seems to hide normal variables
successfully. This needs more investigation.

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->

Allows recursive constant exports from submodules.

# Tests + Formatting
<!--
Don't forget to add tests that cover your changes.

Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` to check that
you're using the standard code style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- -c "use std testing; testing run-tests --path
crates/nu-std"` to run the tests for the standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->