FPHSA: detect package name mismatches

The _FPHSA_NAME_MISMATCHED variable may be set if this is intentional (but should be cleared after the call to not affect other FPHSA calls).

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From https://discourse.cmake.org/t/the-nightmare-of-copy-paste-of-findxxx-cmake-via-the-case-of-libvorbis/334/2