[release/7.0] Fix relative symlink support in TarFile

[github-actions]

Backport of #77338 to release/7.0

/cc @jozkee@am11

Customer Impact

• Symlinks having as target a relative path were always created with an absolute path on extraction.
• Symlinks having as target a relative path pointing to a different directory could throw an exception if Path.Combine(destinationDirectory, path) resulted in a path outside of the destination directory. Said Path combination was incorrect since links targets are relative to where the link is located.

With this fix, symlinks with relative targets don’t run into unexpected exceptions and their relative target is correctly preserved on extraction.
It is an important scenario as relative symlinks are perhaps the most common kind of symlink for their portability and both issues stated above would mess with that.

Testing

Added tests for relative targets pointing inside (OK) and outside (Throw) the destination directory, combined with the link being in multiple levels of the destination directory.

Risk

Low, System.Formats.Tar is a new feature in 7.0

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Issue Details

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Backport of #77338 to release/7.0

/cc @jozkee@am11

Customer Impact

Testing

Risk

IMPORTANT: Is this backport for a servicing release? If so and this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

Author:	github-actions[bot]
Assignees:	-
Labels:	area-System.IO
Milestone:	-

[carlossanlop]

Approved by Tactics via email. CI is green. Approved by area owner. No OOB package authoring changes needed.
Will merge when the branch reopens to take servicing changes on Nov. 29th.

[carlossanlop]

Branding has been done. Milestone is 7.0.2. CI is green. Signed-off by area owners. Approved by Tactics. No OOB package authoring changes needed. Ready to merge.