[Sofa.Config] FIX nested relocatable

[guparan]

WARNING: this will break SofaPython3. I'm working on it.

Before, when 2 nested packages had RELOCATABLE "plugins", the 2nd was installed in CMAKE_INSTALL_PREFIX/plugins/FirstPackage/plugins/SecondPackage.

Now, the RELOCATABLE argument, if relative, will always be treated as CMAKE_INSTALL_PREFIX/.

---

By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

---

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).

[guparan]

[ci-build]