Core implementation of the `@require` macro and supporting functions. Defines `listenpkg` to register callbacks for package load events, `loadpkg` to invoke them when a package is loaded, and `withpath`/`err` for safe execution with proper error handling and source path tracking. The `replace_include` function transforms `include` calls inside `@require` blocks to use the cached `@include` macro. This module is self-contained, imports only from Base and UUIDs, and can be extracted as a standalone pattern for any Julia package that needs conditional loading.

Defines the `@include` macro, which behaves like `include` but caches the file content at macro expansion time. At runtime, if the file still exists, it includes normally; otherwise, it falls back to the cached string. This is useful for building system images where files may not be present at runtime. The macro is used automatically by `@require` when `include` is called inside a require block. The pattern is reusable for any Julia project that needs to embed file content at compile time.