Allow symlinks in the build backend (#14212)
In workspaces with multiple packages, you usually don't want to include
shared files such as the license repeatedly. Instead, we reading from
symlinked files. This would be supported if we had used std's `is_file`
and read methods, but walkdir's `is_file` does not consider symlinked
files as files.
See https://github.com/astral-sh/uv/issues/3957#issuecomment-2994675003
