Require uv venv --clear before removing an existing directory (#14309)

By default, `uv venv <venv-name>` currently removes the `<venv-name`>
directory if it exists. This can be surprising behavior: not everyone
expects an existing environment to be overwritten. This PR updates the
default to fail if a non-empty `<venv-name>` directory already exists
and neither `--allow-existing` nor the new `-c/--clear` option is
provided (if a TTY is detected, it prompts first). If it's not a TTY,
then uv will only warn and not fail for now — we'll make this an error
in the future. I've also added a corresponding `UV_VENV_CLEAR` env var.

I've chosen to use `--clear` instead of `--force` for this option
because it is used by the `venv` module and `virtualenv` and will be
familiar to users. I also think its meaning is clearer in this context
than `--force` (which could plausibly mean force overwrite just the
virtual environment files, which is what our current `--allow-existing`
option does).

Closes #1472.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

.github/workflows/ci.yml

@@ -1042,7 +1042,7 @@ jobs:
 
       - name: "Create a virtual environment (uv)"
         run: |
-          ./uv venv -p 3.13t --managed-python
+          ./uv venv -c -p 3.13t --managed-python
 
       - name: "Check version (uv)"
         run: |
@@ -1087,7 +1087,7 @@ jobs:
 
       - name: "Create a virtual environment (uv)"
         run: |
-          ./uv venv -p 3.13 --managed-python
+          ./uv venv -c -p 3.13 --managed-python
 
       - name: "Check version (uv)"
         run: |
@@ -1132,7 +1132,7 @@ jobs:
 
       - name: "Create a virtual environment (uv)"
         run: |
-          ./uv venv -p 3.13 --managed-python
+          ./uv venv -c -p 3.13 --managed-python
 
       - name: "Check version (uv)"
         run: |
@@ -1758,14 +1758,14 @@ jobs:
           ./uv run --no-project python -c "from built_by_uv import greet; print(greet())"
 
           # Test both `build_wheel` and `build_sdist` through uv
-          ./uv venv -v
+          ./uv venv -c -v
           ./uv build -v --force-pep517 scripts/packages/built-by-uv --find-links crates/uv-build/dist --offline
           ./uv pip install -v scripts/packages/built-by-uv/dist/*.tar.gz --find-links crates/uv-build/dist --offline --no-deps
           ./uv run --no-project python -c "from built_by_uv import greet; print(greet())"
 
           # Test both `build_wheel` and `build_sdist` through the official `build`
           rm -rf scripts/packages/built-by-uv/dist/
-          ./uv venv -v
+          ./uv venv -c -v
           ./uv pip install build
           # Add the uv binary to PATH for `build` to find
           PATH="$(pwd):$PATH" UV_OFFLINE=1 UV_FIND_LINKS=crates/uv-build/dist ./uv run --no-project python -m build -v --installer uv scripts/packages/built-by-uv