Rework --clear flag for venv command

[flyaroundme]

Summary

Reworked the --clear flag for venv command for it not to recreate the existing virtualenv unless --clear flag is set.

Based on this discussion #1472

Test Plan

Create venv using uv venv command. Then use uv venv command again without flag. Check that virtualenv is not cleared. Then use uv venv --clear, check that it is recreated.

$ uv venv
Using Python 3.10.12 interpreter at /usr/bin/python3.10
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate
$ uv pip install ruff # fill virtualenv with something
Resolved 1 package in 141ms
Installed 1 package in 21ms
 + ruff==0.2.2
$ uv pip freeze # memorize the previous state of virtualenv
ruff==0.2.2
$ uv venv # without flag
Using Python 3.10.12 interpreter at /usr/bin/python3.10
Virtualenv already exists at .venv. Use --clear option to recreate
$ uv pip freeze # check that virtualenv is not cleared
ruff==0.2.2
$ uv venv --clear # with flag
Using Python 3.10.12 interpreter at /usr/bin/python3.10
Creating virtualenv at: .venv
Activate with: source .venv/bin/activate
$ uv pip freeze # check virtualenv is cleared
$

[T-256]

I'd rather to fail with non-zero exit code.
Also, message could be more strict since the command failing:

Failed to create virtualenv at '{}': Already exists.
hint: Use --clear option to force recreate new virtualenv at {}

[T-256]

IMO, We could also consider --force instead of --clear to also fix #1863 where target path is exists but is not venv.

[zanieb]

Quite a bit of discussion about this over in #2548