src layout option

[kamathramu87]

PR Checklist

• A description of the changes is added to the description of this PR.
• If there is a related issue, make sure it is linked to this PR.
• If you've fixed a bug or added code that should be tested, add tests!
• Documentation in docs is updated

Description of changes
Some teams prefer using src layout for the project. Benefit of using src layout are described here

[fpgmaas]

This currently breaks functionality if the directory already exists:

Traceback (most recent call last):
  File "/var/folders/dk/4hmjslc11rggp3bqq8tnjv4w0000gn/T/tmp6j1_71zf.py", line 49, in <module>
    move_to_src_layout()
  File "/var/folders/dk/4hmjslc11rggp3bqq8tnjv4w0000gn/T/tmp6j1_71zf.py", line 24, in move_to_src_layout
    src_dir.rename(tgt_dir / src_dir.name)
  File "/Users/florian/.pyenv/versions/3.11.2/lib/python3.11/pathlib.py", line 1175, in rename
    os.rename(self, target)
OSError: [Errno 66] Directory not empty: '/Users/florian/git/cookiecutter-poetry/example-project/example_project' -> '/Users/florian/git/cookiecutter-poetry/example-project/src/example_project'
ERROR: Stopping generation because post_gen_project hook script didn't exit successfully
Hook script failed (exit status: 1)
make: *** [bake-with-inputs] Error 1

To make life easier, maybe we can simply create both src/{{cookiecutter.project_slug}} and {{cookiecutter.project_slug}}, and just delete the one we do not need in the post hook.

Also, currently the y/n defaults to y, so many users who used this template earlier will now get some unexpected results. Maybe instead of a y/n prompt, let's prompt for layout with options flat or src, and default to flat.

[fpgmaas]

Additionally, the behavior/config for mypy is probably different. We might need to add a unit test that checks if mypy still runs successfully in both cases.