feat: add PackageUpgrader interface for targeted package upgrades

Add optional PackageUpgrader interface with UpgradePackages method
that upgrades specific installed packages (unlike Upgrade which
upgrades everything).

Each backend uses native upgrade commands:
- apt: apt-get install --only-upgrade
- dnf: dnf upgrade <pkg>
- pacman: pacman -S <pkg>
- apk: apk upgrade <pkg>
- pkg (FreeBSD): pkg upgrade <pkg>
- flatpak: flatpak update <pkg>
- snap: snap refresh <pkg>

Non-installed packages are filtered out and returned as Unchanged.

Closes #31

pacman/pacman.go

@@ -85,3 +85,11 @@ func (p *Pacman) Version(ctx context.Context, pkg string) (string, error) {
 
 // Verify interface compliance at compile time.
 var _ snack.Manager = (*Pacman)(nil)
+var _ snack.PackageUpgrader = (*Pacman)(nil)
+
+// UpgradePackages upgrades specific installed packages.
+func (p *Pacman) UpgradePackages(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) (snack.InstallResult, error) {
+	p.Lock()
+	defer p.Unlock()
+	return upgradePackages(ctx, pkgs, opts...)
+}

pacman/pacman_linux.go

@@ -219,3 +219,36 @@ func version(ctx context.Context, pkg string) (string, error) {
 	}
 	return parts[1], nil
 }
+
+func upgradePackages(ctx context.Context, pkgs []snack.Target, opts ...snack.Option) (snack.InstallResult, error) {
+	o := snack.ApplyOptions(opts...)
+	var toUpgrade []snack.Target
+	var unchanged []string
+	for _, t := range pkgs {
+		if o.DryRun {
+			toUpgrade = append(toUpgrade, t)
+			continue
+		}
+		ok, err := isInstalled(ctx, t.Name)
+		if err != nil {
+			return snack.InstallResult{}, err
+		}
+		if !ok {
+			unchanged = append(unchanged, t.Name)
+		} else {
+			toUpgrade = append(toUpgrade, t)
+		}
+	}
+	if len(toUpgrade) > 0 {
+		args := append([]string{"-S"}, formatTargets(toUpgrade)...)
+		if _, err := run(ctx, args, o); err != nil {
+			return snack.InstallResult{}, err
+		}
+	}
+	var upgraded []snack.Package
+	for _, t := range toUpgrade {
+		v, _ := version(ctx, t.Name)
+		upgraded = append(upgraded, snack.Package{Name: t.Name, Version: v, Installed: true})
+	}
+	return snack.InstallResult{Installed: upgraded, Unchanged: unchanged}, nil
+}

pacman/pacman_other.go

@@ -49,3 +49,7 @@ func isInstalled(_ context.Context, _ string) (bool, error) {
 func version(_ context.Context, _ string) (string, error) {
 	return "", snack.ErrUnsupportedPlatform
 }
+
+func upgradePackages(_ context.Context, _ []snack.Target, _ ...snack.Option) (snack.InstallResult, error) {
+	return snack.InstallResult{}, snack.ErrUnsupportedPlatform
+}