Default to PEP 517-based builds (#843)

## Summary

Our current setup uses the legacy `setup.py`-based builds if a
`pyproject.toml` file isn't present. This matches pip's behavior.
However, `pypa/build` uses PEP 517-based builds in such cases, and it
looks like pip plans to make that the default
(https://github.com/pypa/pip/issues/9175), with the limiting factor
being performance issues related to isolated builds.

This is now the default behavior, but the `--legacy-setup-py` flag
allows users to opt-in to using `setup.py` directly for distributions
that lack a `pyproject.toml`.

crates/puffin-dev/src/resolve_cli.rs

@@ -17,6 +17,7 @@ use puffin_client::RegistryClientBuilder;
 use puffin_dispatch::BuildDispatch;
 use puffin_interpreter::Virtualenv;
 use puffin_resolver::{Manifest, ResolutionOptions, Resolver};
+use puffin_traits::SetupPyStrategy;
 
 #[derive(ValueEnum, Default, Clone)]
 pub(crate) enum ResolveCliFormat {
@@ -50,6 +51,7 @@ pub(crate) async fn resolve_cli(args: ResolveCliArgs) -> Result<()> {
     let venv = Virtualenv::from_env(platform, &cache)?;
     let client = RegistryClientBuilder::new(cache.clone()).build();
     let index_urls = IndexUrls::default();
+    let setup_py = SetupPyStrategy::default();
 
     let build_dispatch = BuildDispatch::new(
         &client,
@@ -57,6 +59,7 @@ pub(crate) async fn resolve_cli(args: ResolveCliArgs) -> Result<()> {
         venv.interpreter(),
         &index_urls,
         venv.python_executable(),
+        setup_py,
         args.no_build,
     );