Add prerelease compatibility check (#8020)
## Summary
Closes #7977. Makes `PythonDownloadRequest` account for the prerelease
part if allowed. Also stores the prerelease in `PythonInstallationKey`
directly as a `Prerelease` rather than a string.
## Test Plan
Correctly picks the relevant prerelease (rather than picking the most
recent one):
```
λ cargo run python install 3.13.0rc2
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
Running `target/debug/uv python install 3.13.0rc2`
Searching for Python versions matching: Python 3.13rc2
cpython-3.13.0rc2-macos-aarch64-none ------------------------------ 457.81 KiB/14.73 MiB ^C
λ cargo run python install 3.13.0rc3
Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.17s
Running `target/debug/uv python install 3.13.0rc3`
Searching for Python versions matching: Python 3.13rc3
Found existing installation for Python 3.13rc3: cpython-3.13.0rc3-macos-aarch64-none
```
