IO-845 PathUtils.copyDirectory copies or follows symbolic links

[peterdemaeyer]

Defined the behavior of PathUtils.copyDirectory for symbolic links. It depends on the option LinkOption.NOFOLLOW_LINKS. Non-symbolic (hard) links are out of scope, given that the copy behavior for them was already well-defined: they're treated as regular files.
For the sake of conciseness, the following paragraphs use the term "link" for "symbolic link".

Without NOFOLLOW_LINKS option:

• Links are followed, so that there are no links in the target directory.
• Cyclic links result in FileSystemLoopException.
• Broken links are ignored, they are not copied at all.

With NOFOLLOW_LINKS option:

• Links are copied as links, so that there are links in the target directory for every link in the source directory.
• Links that link to a file or directory inside the source directory are copied to relative links inside the target directory, linking to the copied file or directory.
• Links that link to a file or directory outside the source directory are copied to links inside the target directory, linking to the same file or directory as the link in the source directory.
• Cyclic links are preserved, mirrored to cyclic links in the target directory.
• Broken links are ignored, they are not copied at all.

All of this is also explained in the updated JavaDoc.

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