[perf] skip forcing printDevEnv when add/rm/update outside shellenv

[savil]

Summary

In this PR, we skip forcing computeNixEnv if a user is adding/removing/updating packages, AND if we are not in a shellenv-enabled environment from that devbox project.

Also:

• rename ensurePackagesAreInstalled to ensureDevboxEnvIsUpToDate
• some minor comment cleanups

How was it tested?

did devbox add vim when:

1. in devbox project's directory, but direnv disabled => did not print "Recomputing Devbox environment."
2. in devbox project's directory with direnv enabled => did print "Recomputing Devbox environment".
3. in devbox shell => did print again

Repeated the same exercise with devbox global:

1. with devbox global shellenv | source run in shellrc => did print "Recomputing Devbox environment".
2. after commenting out that line in shellrc => did not print
   NOTE: there's no special devbox-global specific code change here.

[savil]

Current dependencies on/for this PR:

• main
  • PR [perf] skip forcing printDevEnv when add/rm/update outside shellenv #1540 👈
    • PR [perf] in syncPackagesToProfile, only removeExtraItemsFromProfile is mode is not install #1541

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[savil]

I forgot to mark this as Ready for Review on Friday!