The hooks that go in install files -- including hook_schema, hook_update_N, hook_modules_un/installed/en/disabled, should state whether the hook code has access to the module code and whether hooks are available to invoke.
As far as I can tell:
- hook_schema: depends on whether the module is being installed or uninstalled
- hook_modules_installed/enabled: has access to the module files and hooks from both the implementer module and the acting modules.
- hook_modules_uninstalled/disabled: has access to no module files or hooks. Module files can be loaded explicitly with drupal_load().