## Summary

Previously, devbox would indicate that an uninstall happened even if a
package wasn't part of the `devbox.json` packages. This isn't great,
especially if a user made an inadvertent typo.

This PR prints a warning for such a scenario.

It also changes logic to print the confirmation messages outside `devbox
shell`.

## How was it tested?

```
> cd testdata/rust/rust-stable
> devbox shell
(devbox)> devbox add go_1_18 nodejs

# package doesn't exist
(devbox)> devbox rm php
Warning: the following packages were not found in your devbox.json: php
Uninstalling nix packages. This may take a while...done.
No packages removed.

# subset of packages exist 
(devbox) > devbox rm php rummy nodejs
Warning: the following packages were not found in your devbox.json: php, rummy
Uninstalling nix packages. This may take a while...done.
nodejs is now removed. Run `hash -r` to ensure your shell is updated.

# package exists
(devbox)> devbox rm go_1_18
Uninstalling nix packages. This may take a while...done.
go_1_18 is now removed. Run `hash -r` to ensure your shell is updated.

```