Read Python version files during toolchain installs (#4335)
A bare `uv toolchain install` invocation now reads default requests from
Python version files in the working directory. In order, a bare
invocation means:
- requests from `.python-versions`
- a single request from`.python-version`
- any installed managed toolchain
- the latest managed toolchain download
This replaces all the functionality of `cargo dev fetch-python`, which
we drop in #4337
