Search for all `python3.x` minor versions in PATH, skipping those we
already know we can use.
For example, let's say `python` and `python3` are Python 3.10. When a
user requests `>= 3.11`, we still need to find a `python3.12` in PATH.
We do so with a regex matcher.
Fixes#4709
## Summary
Closes https://github.com/astral-sh/uv/issues/4915.
```
error: No interpreter found for Python 3.12.4 in virtual environments, managed installations, or system path
```
Whew this is a lot.
The user-facing changes are:
- `uv toolchain` to `uv python` e.g. `uv python find`, `uv python
install`, ...
- `UV_TOOLCHAIN_DIR` to` UV_PYTHON_INSTALL_DIR`
- `<UV_STATE_DIR>/toolchains` to `<UV_STATE_DIR>/python` (with
[automatic
migration](https://github.com/astral-sh/uv/pull/4735/files#r1663029330))
- User-facing messages no longer refer to toolchains, instead using
"Python", "Python versions" or "Python installations"
The internal changes are:
- `uv-toolchain` crate to `uv-python`
- `Toolchain` no longer referenced in type names
- Dropped unused `SystemPython` type (previously replaced)
- Clarified the type names for "managed Python installations"
- (more little things)
konsti