fix copying top level symlinks to folders in directory unpack_strategy

[chrmoritz]

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This PR fixes a issue in the directory unpack_strategy, which currently does not preserve top level symlinks pointing to directories. Instead of copying the symlink (as a singe file) we are currently recursively copying the content of the folder to which the symlink points to. At the end the unpacked source does not contain the top level symlink (pointing to a folder within the source) anymore, because brew has converted it to a complete copy of said folder.

This breaks deno (see: Homebrew/homebrew-core#35645 (comment)) and the added test case.

The current code is broken because child.directory? is true for symlinks pointing to folders too, but we should preserve them. That's why an additional check if the child is not a symlink was added to determine if we should recursively copy the contents of the child (in case it's a non symlink folder) or just the file itself.

[MikeMcQuaid]

Great work as usual, thanks @chrmoritz!