Closes https://github.com/astral-sh/uv/issues/3857
Instead of using custom `Arch`, `Os`, and `Libc` types I just use
`target-lexicon`'s which enumerate way more variants and implement
display and parsing. We use a wrapper type to represent a couple special
cases to support the "x86" alias for "i686" and "macos" for "darwin".
Alternatively we could try to use our `platform-tags` types but those
capture more information (like operating system versions) that we don't
have for downloads.
As discussed in https://github.com/astral-sh/uv/pull/4160, this is not
sufficient for proper libc detection but that work is larger and will be
handled separately.
Zanie Blue