In preparation for managed toolchains #2607, just renames the crate to
something broader.

See #4121 and #4138 to see the final
intent.

Extends #4121

…4138)

Extends #4121
Part of #2607 

Adds support for managed toolchain fetching to `uv venv`, e.g.

```
❯ cargo run -q -- venv --python 3.9.18 --preview -v
DEBUG Searching for Python 3.9.18 in search path or managed toolchains
DEBUG Searching for managed toolchains at `/Users/zb/Library/Application Support/uv/toolchains`
DEBUG Found CPython 3.12.3 at `/opt/homebrew/bin/python3` (search path)
DEBUG Found CPython 3.9.6 at `/usr/bin/python3` (search path)
DEBUG Found CPython 3.12.3 at `/opt/homebrew/bin/python3` (search path)
DEBUG Requested Python not found, checking for available download...
DEBUG Using registry request timeout of 30s
INFO Fetching requested toolchain...
DEBUG Downloading https://github.com/indygreg/python-build-standalone/releases/download/20240224/cpython-3.9.18%2B20240224-aarch64-apple-darwin-pgo%2Blto-full.tar.zst to temporary location /Users/zb/Library/Application Support/uv/toolchains/.tmpgohKwp
DEBUG Extracting cpython-3.9.18%2B20240224-aarch64-apple-darwin-pgo%2Blto-full.tar.zst
DEBUG Moving /Users/zb/Library/Application Support/uv/toolchains/.tmpgohKwp/python to /Users/zb/Library/Application Support/uv/toolchains/cpython-3.9.18-macos-aarch64-none
Using Python 3.9.18 interpreter at: /Users/zb/Library/Application Support/uv/toolchains/cpython-3.9.18-macos-aarch64-none/install/bin/python3
Creating virtualenv at: .venv
INFO Removing existing directory
Activate with: source .venv/bin/activate
```

The preview flag is required. The fetch is performed if we can't find an
interpreter that satisfies the request. Once fetched, the toolchain will
be available for later invocations that include the `--preview` flag.
There will be follow-ups to improve toolchain management in general,
there is still outstanding work from the initial implementation.