Fix registry download with symlinks in root

[fortmarek]

Fix registry download with symlinks in the root

Motivation:

You can find the reproducer here: https://github.com/fortmarek/symlink-swiftpm-reproducer/tree/main

When running the following:

• rm -rf ~/.swiftpm .build && swift package purge-cache
• followed with swift package --replace-scm-with-registry resolve

The resolution fails with:

error: 'tuist.Noora': failed downloading tuist.Noora version 0.49.0 source archive from https://tuist.dev/api/registry/swift: failed extracting '/Users/marekfort/Library/Caches/org.swift.swiftpm/registry/downloads/tuist/Noora/0.49.0.zip' to '/Users/marekfort/Library/Caches/org.swift.swiftpm/registry/downloads/tuist/Noora/0.49.0': /Users/marekfort/Library/Caches/org.swift.swiftpm/registry/downloads/tuist/Noora/A76E901C-26CA-4A7B-BAC7-97ABA638F109/CLAUDE.md doesn't exist in file system
error: fatalError

This is because this particular version of the library has the following files in the root directory:

• AGENT.md
• CLAUDE.md symlink pointing to AGENT.md

The error occurs here when moving the downloaded source as part of the stripFirstLevel method where files are moved one-by-one. But when AGENT.md is returned as the first file from the getDirectoryContents, the subsequent move of CLAUDE.md fails because the move method of FileSystem checks whether a file exists – but in the case of symlinks, it follows them, and since the CLAUDE.md symlink points to AGENT.md that was already moved, the method throws an error.

Modifications:

The fix is in the swift-tools-support-core library by changing the exists method to not follow symlinks: swiftlang/swift-tools-support-core#530

Note this PR will be invalid until that PR is merged.

Result:

Following the repro steps from the motivation results in a successful SwiftPM resolution.