[Task] Clean up `add_imported_deps` function in imports module

The match statement in `add_imported_deps` function located in `import.rs` itself should turn into something like:
```rs
// The `publics` here is a vector of items implementing `Declaration` trait which comes with `get_name`
let found = publics.iter_mut().find(|decl| decl.get_name() == name);

match found {
Some(Function(fun)) => ...,
Some(Variable(var)) => ...,
None => // "Import '{name}' not found"
}
```
This requires:
1. Implementation of a `Declaration` enum
2. Restructuring the code in the match statement
3. Update `get_imports` in the `import_cache.rs` to use the new `Declaration` enum