Change "toolchain" to "python" (#4735)

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)

crates/uv-python/template-download-metadata.py

@@ -0,0 +1,128 @@
+#!/usr/bin/env python3.12
+"""
+Generate static Rust code from Python version download metadata.
+
+Generates the `downloads.inc` file from the `downloads.inc.mustache` template.
+
+Usage:
+
+    uv run --isolated --with chevron-blue -- crates/uv-python/template-download-metadata.py
+"""
+
+import sys
+import logging
+import argparse
+import json
+import subprocess
+from pathlib import Path
+
+CRATE_ROOT = Path(__file__).parent
+WORKSPACE_ROOT = CRATE_ROOT.parent.parent
+VERSION_METADATA = CRATE_ROOT / "download-metadata.json"
+TEMPLATE = CRATE_ROOT / "src" / "downloads.inc.mustache"
+TARGET = TEMPLATE.with_suffix("")
+
+
+try:
+    import chevron_blue
+except ImportError:
+    print(
+        "missing requirement `chevron-blue`",
+        file=sys.stderr,
+    )
+    exit(1)
+
+
+def prepare_name(name: str) -> str:
+    match name:
+        case "cpython":
+            return "CPython"
+        case _:
+            raise ValueError(f"Unknown implementation name: {name}")
+
+
+def prepare_libc(libc: str) -> str | None:
+    if libc == "none":
+        return None
+    else:
+        return libc.title()
+
+
+def prepare_arch(arch: str) -> str:
+    match arch:
+        # Special constructors
+        case "i686":
+            return "X86_32(target_lexicon::X86_32Architecture::I686)"
+        case "aarch64":
+            return "Aarch64(target_lexicon::Aarch64Architecture::Aarch64)"
+        case "armv7":
+            return "Arm(target_lexicon::ArmArchitecture::Armv7)"
+        case _:
+            return arch.capitalize()
+
+
+def prepare_value(value: dict) -> dict:
+    value["os"] = value["os"].title()
+    value["arch"] = prepare_arch(value["arch"])
+    value["name"] = prepare_name(value["name"])
+    value["libc"] = prepare_libc(value["libc"])
+    return value
+
+
+def main():
+    debug = logging.getLogger().getEffectiveLevel() <= logging.DEBUG
+
+    data = {}
+    data["generated_with"] = Path(__file__).relative_to(WORKSPACE_ROOT)
+    data["generated_from"] = TEMPLATE.relative_to(WORKSPACE_ROOT)
+    data["versions"] = [
+        {"key": key, "value": prepare_value(value)}
+        for key, value in json.loads(VERSION_METADATA.read_text()).items()
+    ]
+
+    # Render the template
+    logging.info(f"Rendering `{TEMPLATE.name}`...")
+    output = chevron_blue.render(
+        template=TEMPLATE.read_text(), data=data, no_escape=True, warn=debug
+    )
+
+    # Update the file
+    logging.info(f"Updating `{TARGET}`...")
+    TARGET.write_text(output)
+    subprocess.check_call(
+        ["rustfmt", str(TARGET)],
+        stderr=subprocess.STDOUT,
+        stdout=sys.stderr if debug else subprocess.DEVNULL,
+    )
+
+    logging.info("Done!")
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser(
+        description="Generates Rust code for Python version metadata.",
+    )
+    parser.add_argument(
+        "-v",
+        "--verbose",
+        action="store_true",
+        help="Enable debug logging",
+    )
+    parser.add_argument(
+        "-q",
+        "--quiet",
+        action="store_true",
+        help="Disable logging",
+    )
+
+    args = parser.parse_args()
+    if args.quiet:
+        log_level = logging.CRITICAL
+    elif args.verbose:
+        log_level = logging.DEBUG
+    else:
+        log_level = logging.INFO
+
+    logging.basicConfig(level=log_level, format="%(message)s")
+
+    main()