go/ast: deprecate FilterPackage, PackageExports, MergePackageFiles

The Go standard library has a package called go/ast that represents Go source code as a tree of nodes. In older versions of the library, a type named ast.Package was used to hold a whole Go package, and several helper functions – FilterPackage, PackageExports, and MergePackageFiles – operated on that type. Those functions are now marked as deprecated, meaning they are still available but are discouraged from use because the type they rely on, ast.Package, is itself deprecated. The change does not add new features; it simply signals that developers should use newer alternatives that work with individual ast.File objects instead.