[all] Fix tinyxml2 dependency

[bakpaul]

This PR aims at correcting bugs related to the newly externalized tinyxml2 dependency in two ways :

1. For Windows, this corrects the FindTinyxml2.cmake to be able to find the lib and install it correctly when using the tinyxml2 lib present in the WinDepPack.
2. For MacOS, this PR correct the rpath of the plugins linking to tinyxml2 for packaging

After this PR is closed, the installed version of tinyxml2 present on the CI builders will be remoced, and the one in the WinDepPack will be used instead (thanks to this pr: https://github.com/sofa-framework/ci/pull/48 and some manual configuration of the builders).

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).