[ls] return unresolved packages as a warning instead of an error (#2511)

## Summary

Fixes #2510 

Previously, `devbox ls --outdated` would error on packages that it could
not resolve, like `stdenv.cc.cc.lib` or
`darwin.apple_sdk.frameworks.IOKit`. This prevented devbox from checking
if the rest of the packages were outdated.

This PR changes the error to a warning, so the rest of the version
checks can proceed

## How was it tested?

1. Create a devbox.json
2. Add standard packages using `devbox add`
3. Add `stdenv.cc.cc.lib` or `darwin.apple_sdk.frameworks.IOKit` to the
project
4. Verify that `devbox ls --outdated` shows a warning for the
non-versioned packages instead of an error.

Author: Lagoja

internal/devbox/packages.go

@@ -52,6 +52,7 @@ type UpdateVersion struct {
 func (d *Devbox) Outdated(ctx context.Context) (map[string]UpdateVersion, error) {
 	lockfile := d.Lockfile()
 	outdatedPackages := map[string]UpdateVersion{}
+	var warnings []string
 
 	for _, pkg := range d.AllPackages() {
 		// For non-devbox packages, like flakes, we can skip for now
@@ -61,7 +62,8 @@ func (d *Devbox) Outdated(ctx context.Context) (map[string]UpdateVersion, error)
 
 		lockPackage, err := lockfile.FetchResolvedPackage(pkg.Versioned())
 		if err != nil {
-			return nil, errors.Wrap(err, "failed to fetch resolved package")
+			warnings = append(warnings, fmt.Sprintf("Note: unable to check updates for %s", pkg.CanonicalName()))
+			continue
 		}
 		existingLockPackage := lockfile.Packages[pkg.Raw]
 		if lockPackage.Version == existingLockPackage.Version {
@@ -71,6 +73,10 @@ func (d *Devbox) Outdated(ctx context.Context) (map[string]UpdateVersion, error)
 		outdatedPackages[pkg.Versioned()] = UpdateVersion{Current: existingLockPackage.Version, Latest: lockPackage.Version}
 	}
 
+	for _, warning := range warnings {
+		fmt.Fprintf(d.stderr, "%s\n", warning)
+	}
+
 	return outdatedPackages, nil
 }