Make < exclusive for non-prerelease markers (#1878)
## Summary
Even when pre-releases are "allowed", per PEP 440, `pydantic<2.0.0`
should _not_ include pre-releases. This PR modifies the specifier
translation to treat `pydantic<2.0.0` as `pydantic<2.0.0.min0`, where
`min` is an internal-only version segment that's invisible to users.
Closes https://github.com/astral-sh/uv/issues/1641.
