Refactor distribution types to adhere to a clear hierarchy (#369)
## Summary
This PR refactors our `RemoteDistribution` type such that it now follows
a clear hierarchy that matches the actual variants, and encodes the
differences between source and built distributions:
```rust
pub enum Distribution {
Built(BuiltDistribution),
Source(SourceDistribution),
}
pub enum BuiltDistribution {
Registry(RegistryBuiltDistribution),
DirectUrl(DirectUrlBuiltDistribution),
}
pub enum SourceDistribution {
Registry(RegistrySourceDistribution),
DirectUrl(DirectUrlSourceDistribution),
Git(GitSourceDistribution),
}
/// A built distribution (wheel) that exists in a registry, like `PyPI`.
pub struct RegistryBuiltDistribution {
pub name: PackageName,
pub version: Version,
pub file: File,
}
/// A built distribution (wheel) that exists at an arbitrary URL.
pub struct DirectUrlBuiltDistribution {
pub name: PackageName,
pub url: Url,
}
/// A source distribution that exists in a registry, like `PyPI`.
pub struct RegistrySourceDistribution {
pub name: PackageName,
pub version: Version,
pub file: File,
}
/// A source distribution that exists at an arbitrary URL.
pub struct DirectUrlSourceDistribution {
pub name: PackageName,
pub url: Url,
}
/// A source distribution that exists in a Git repository.
pub struct GitSourceDistribution {
pub name: PackageName,
pub url: Url,
}
```
Most of the PR just stems downstream from this change. There are no
behavioral changes, so I'm largely relying on lint, tests, and the
compiler for correctness.
