use unreachable keyword for unreachable code path

Author: ianic

src/Package/Fetch.zig

@@ -1184,7 +1184,7 @@ fn unpackTarball(f: *Fetch, out_dir: fs.Dir, reader: anytype) RunError!UnpackRes
                 .unable_to_create_file => |i| res.unableToCreateFile(stripRoot(i.file_name, res.root_dir), i.code),
                 .unable_to_create_sym_link => |i| res.unableToCreateSymLink(stripRoot(i.file_name, res.root_dir), i.link_name, i.code),
                 .unsupported_file_type => |i| res.unsupportedFileType(stripRoot(i.file_name, res.root_dir), @intFromEnum(i.file_type)),
-                .components_outside_stripped_prefix => {}, // impossible with strip_components = 0
+                .components_outside_stripped_prefix => unreachable, // unreachable with strip_components = 0
             }
         }
     }