feat(apt,dpkg): implement capability interfaces

apt: VersionQuerier, Holder, Cleaner, FileOwner, RepoManager, KeyManager, NameNormalizer
dpkg: FileOwner, NameNormalizer

All new interfaces follow the existing pattern of exported methods on
struct delegating to unexported platform-specific functions. Mutating
operations use Lock/Unlock; read-only operations do not.

Platform stubs in _other.go files return ErrUnsupportedPlatform.
Compile-time interface checks added for all implemented interfaces.

dpkg/normalize.go

@@ -0,0 +1,21 @@
+package dpkg
+
+import "strings"
+
+func normalizeName(name string) string {
+	n, _ := parseArch(name)
+	return n
+}
+
+func parseArch(name string) (string, string) {
+	if idx := strings.LastIndex(name, ":"); idx >= 0 {
+		pkg := name[:idx]
+		arch := name[idx+1:]
+		switch arch {
+		case "amd64", "i386", "arm64", "armhf", "armel", "mips", "mipsel",
+			"mips64el", "ppc64el", "s390x", "all", "any":
+			return pkg, arch
+		}
+	}
+	return name, ""
+}