Rename Virtualenv and PythonPlatform structs (#2034)

## Summary

`PythonPlatform` only exists to format paths to directories within
virtual environments based on a root and an OS, so it's now
`VirtualenvLayout`.

`Virtualenv` is now used for non-virtual environment Pythons, so it's
now `PythonEnvironment`.

crates/uv-dev/src/resolve_cli.rs

@@ -16,7 +16,7 @@ use uv_cache::{Cache, CacheArgs};
 use uv_client::{FlatIndex, FlatIndexClient, RegistryClientBuilder};
 use uv_dispatch::BuildDispatch;
 use uv_installer::NoBinary;
-use uv_interpreter::Virtualenv;
+use uv_interpreter::PythonEnvironment;
 use uv_resolver::{InMemoryIndex, Manifest, Options, Resolver};
 use uv_traits::{ConfigSettings, InFlight, NoBuild, SetupPyStrategy};
 
@@ -55,7 +55,7 @@ pub(crate) async fn resolve_cli(args: ResolveCliArgs) -> Result<()> {
     let cache = Cache::try_from(args.cache_args)?;
 
     let platform = Platform::current()?;
-    let venv = Virtualenv::from_env(platform, &cache)?;
+    let venv = PythonEnvironment::from_virtualenv(platform, &cache)?;
     let index_locations = IndexLocations::new(
         Some(args.index_url),
         args.extra_index_url,