TURBOPACK: Fix subpath imports resolving to dependencies

[yuzheng14]

Summary

• Allow package.json subpath imports targets to resolve as bare package specifiers instead of always forcing relative paths.
• Add a snapshot fixture verifying an imports entry can point to a dependency in node_modules.

Upstream PR vercel#93308 is addressing the same issue, but there is no clear timeline on when it will be merged.

Test plan

• cargo test -p turbopack-tests snapshot covers subpath-imports resolving to a node_modules dependency

[gemini-code-assist]

Code Review

This pull request updates Turbopack's resolution logic to support subpath imports by allowing both relative and bare specifier resolutions. It also exposes the ExportImport enum internally and adds a test case for subpath imports within node_modules. Feedback was provided regarding the use of Pattern::alternatives in handle_exports_imports_field, which may introduce ambiguity or deviate from the Node.js specification. The reviewer suggested a refactor to improve readability and ensure spec compliance.

[gemini-code-assist]

Using Pattern::alternatives to try both a relative path (prefixed with ./) and the original path for subpath imports can lead to ambiguous resolution. According to the Node.js specification, subpath import targets are either relative (starting with ./ or ../) or bare package specifiers. Trying both might cause a bare specifier like "#dep": "my-pkg" to accidentally resolve to a local file ./my-pkg if it exists, which deviates from standard Node.js behavior.

If the intention is to strictly follow the spec, result_path.clone() should be sufficient for ExportImport::Import, as Request::parse will correctly handle the relativeness based on the prefix. If backward compatibility for missing ./ prefixes is required, the current approach is a safe fallback, but the code can be simplified to avoid redundant shadowing and improve readability.

            let request = match kind {
                ExportImport::Export => {
                    Pattern::Concatenation(vec![Pattern::Constant(rcstr!("./")), result_path.clone()])
                }
                ExportImport::Import => Pattern::alternatives(vec![
                    Pattern::Concatenation(vec![
                        Pattern::Constant(rcstr!("./")),
                        result_path.clone(),
                    ]),
                    result_path.clone(),
                ]),
            };
            let request = *Request::parse(request).to_resolved().await?;