fix!(core): detect ambiguous imports / declarations

[mttrbrts]

Closes #648

Changes

• Detects when local declarations have the same shortname as an imported type
• Most of the changes here are fixing tests that previously relied on this behaviour

Flags

• The side-effect is that you can no longer declare a type with the same name as a System type (because the system namespace is implicitly imported in all Model files).

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname