[per-OS Packages] Error if adding --platform, when excluded_platforms is defined

[savil]

Summary

Background:
for a package, platforms and excluded_platforms should be defined mutually-exclusively.

Previously, we were being "clever" by dropping "excluded_platforms" when a user
does devbox add <package> --platform <platform>. But I think we should:

1. Ensure the user explicitly makes the change to their devbox config.
2. Be consistent with how the inverse is handled: using --exclude-platform when platform is defined.

How was it tested?

added testscript unit-tests

[savil]

Current dependencies on/for this PR:

• main
  • PR [per-OS Packages] Error if adding --platform, when excluded_platforms is defined #1373 👈

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