Add support for local imports in hooks

[LKajan]

Render the whole hook directory to a temporary folder before executing hooks so that it's possible to use local imports in hooks.

I kindly request comments regarding this implementation before finalizing this PR by adding tests and documentation.

A few notes about the implementation:

• Pre_prompt_hook is still executed from the original location.
• Relative imports in hooks are not supported. This is because relative imports are not possible if the hook module itself is not imported, hooks are executed as scripts.

A few questions that I have:

• How much do we care about the backwards compatibility of the API? This leaves the following methods orphaned:
  • run_hook_from_repo_dir()
  • run_hook()
  • run_script_with_context()
• Should it be possible to modify the post_gen_hook by the pre_gen_hook?
• Should the repo directory be added to the PYTHONPATH during hook execution so that hooks can access packages from the repo directory?
• What is Cookiecutter's policy regarding typing annotations? Should those be added to the new code?

Resolves #824