Warn when `venv` target directory already contains an environment

Feature or enhancement

Proposal:

When a user runs:

python -m venv myenv

and the directory already contains a virtual environment, Python silently performs no action. This behavior can be confusing, as the user receives no indication that the command had no effect.

I propose that venv should display a simple warning when it detects an existing virtual environment so the user knows the operation didn’t make any changes.

Example of the suggested output:

$ python -m venv myenv
Existing virtual environment found at 'myenv'.
No changes were made.
Use '--clear' to recreate this environment.

This provides clarity while keeping behavior backward-compatible. Users who want to recreate the environment can explicitly use --clear.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

从 python -m venv 入口点开始，比较目标目录中已经包含环境时的行为。验证现有的 --clear 选项如何改变该行为。当现有环境产生提议的警告并报告未进行任何更改，同时显式清除仍然可用时，即视为完成。