[per-OS Packages] fix devbox update for unversioned packages with platform/excluded-platform

[savil]

Summary

This PR fixes a slightly edge-casey workflow:

1. User on macOS tries to devbox shell with glibcLocales in devbox.json. Note: unversioned package.
2. Is asked to run: devbox add glibcLocales --exclude-platforms x86_64-darwin
3. Running that, prints a notice to run devbox update.
4. Runs devbox update.

This should work, but was previously broken for two reasons:

1. In update.go, we were calling devbox.Add with nil for platforms and excluded_platforms.
2. In devbox.Add, we check versionedPkg.ValidateExists() prior to converting the raw package name to the versioned-name.
   Both are now fixed.

How was it tested?

Ran the workflow described above. It's hard to write a testscript unit-test, since
some packages build may or may not build on whichever system is running the unit-test. And mocking is too complicated here.

[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 👈

This comment was auto-generated by Graphite.