Allow conflicting prerelease strategies when forking (#5150)
## Summary
Similar to https://github.com/astral-sh/uv/pull/5232, we should also
track prerelease strategies per-fork, instead of globally per package.
The common functionality for tracking locals and prerelease versions
across forks is extracted into the `ForkMap` type.
Resolves https://github.com/astral-sh/uv/issues/4579. This doesn't quite
solve https://github.com/astral-sh/uv/issues/4959, as that issue relies
on overlapping markers.
