devbox.findPackagesByName should be constructed from Config packages

[savil]

Summary

This PR refactors devbox.findPackagesByName to be constructed from devconfig.Package.

Motivation
devbox.Update calls findPackageByName and needs to call devbox.Add again with the Platform and ExcludedPlatforms. (see #1370)

How was it tested?

compiles

[savil]

Current dependencies on/for this PR:

• main
  • PR [Packages] Introduce the concept of Installable packages #1357
    • PR devbox.findPackagesByName should be constructed from Config packages #1360 👈
      • PR [per-OS Packages] Add platforms and excluded_platforms functionality #1358
        • PR [per-OS Packages] in devbox add, allow multiple --platform/--exclude-platform flag values #1363
          • PR [per-OS Packages] Suggest --platform/--exclude-platform upon error with installing package that is uninstallable on user's platform #1364
            • PR [per-OS Packages] fix devbox update for unversioned packages with platform/excluded-platform #1370

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[savil]

All tests pass. Going to land the parent PR, rebase and force-land.