[Config] Fix Findmetis module when using config mode

[olivier-roussel]

Fix several bugs when having a metis package installed and its cmake config files are expected to be used by our Findmetis cmake module file:

• cmake config files provided by metis packages (at least conda, as APT do not provide such) are prefixed with an uppercase (e.g. MetisConfig.cmake instead of metisConfig.cmake), although all cmake variables and targets are then correctly set to lowercase. This will make the library not findable in config mode on case senstitive OS such linux.
• do not fail if invalid version of metis is found on the system, instead allow fetching the right one (is SOFA_ALLOW_FETCH_DEPENDENCIES is ON)

---

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).