[fix] Include nested packages in SysML textual export

[vik378]

Summary

Fixes nested packages being omitted from the SysML textual export output.

Root Cause

In SysMLRelationPredicates.java, the import for org.eclipse.syson.sysml.Package was missing. This caused Package.class to resolve to java.lang.Package instead of the SysML Package type, resulting in nested packages not being recognized during the textual export traversal.

Fix

Added the missing import:

import org.eclipse.syson.sysml.Package;

Testing

• Export model with nested packages
• Verify nested packages appear in textual output
• Verify re-import produces same structure

[AxelRICHARD]

Hello @vik378, thank you for this contribution!
Could you please create an issue and then link this PR to the issue in your commit message? Please read this chapter https://doc.mbse-syson.org/syson/v2026.3.0/developer-guide/index.html#_contribute_a_change_in_the_codebase of the developer guide, it will show you how to do it. You can also take a look at the git history and you well see how we wrote our commit messages.
Furthermore, you deleted the PR guideline, could you please put it back and check all the items?
Thank you.
Regards,

[AxelRICHARD]

Furthermore, if you are not have an Eclipse account and didn't signed the Eclipse ECA I won't be able to merge this PR.
SysON is a project of the Eclipse foundation and only people with an Eclipse account and the signed ECA can have their PRs be merged into this repository.
So you have two choices here: you create an Eclipse account and sign the ECA or you allow me to push your contribution under my name.