[DOC] Roadmap updates

[peytondmurray]

Description

This pull request updates the roadmap:

• Explicitly adds an item for making a plugin system for locking, solving, and package management
• Adds links to issues for a number of items that were missing them

Pull request checklist

• Did you test this change locally?
• Did you update the documentation (if required)?
• Did you add/update relevant tests for this change (if required)?

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