feat: initial project scaffold

- Common Manager interface, Package type, functional options
- Sentinel errors for common package manager failures
- Sub-package stubs for: pacman, aur, apk, apt, dpkg, dnf, rpm,
  flatpak, snap, pkg (FreeBSD), ports (OpenBSD)
- detect/ package for auto-detection of system package manager
- 0BSD license

errors.go

@@ -0,0 +1,31 @@
+package snack
+
+import "errors"
+
+var (
+	// ErrNotInstalled is returned when a queried package is not installed.
+	ErrNotInstalled = errors.New("package is not installed")
+
+	// ErrNotFound is returned when a package cannot be found in any repository.
+	ErrNotFound = errors.New("package not found")
+
+	// ErrUnsupportedPlatform is returned when a package manager is not
+	// available on the current platform.
+	ErrUnsupportedPlatform = errors.New("package manager not available on this platform")
+
+	// ErrPermissionDenied is returned when an operation requires elevated
+	// privileges that were not provided.
+	ErrPermissionDenied = errors.New("permission denied; try WithSudo()")
+
+	// ErrAlreadyInstalled is returned when attempting to install a package
+	// that is already present.
+	ErrAlreadyInstalled = errors.New("package is already installed")
+
+	// ErrDependencyConflict is returned when a package has unresolvable
+	// dependency conflicts.
+	ErrDependencyConflict = errors.New("dependency conflict")
+
+	// ErrManagerNotFound is returned by detect when no supported package
+	// manager can be found on the system.
+	ErrManagerNotFound = errors.New("no supported package manager found")
+)