CMake: Fix case in FindPugixml resulting in false negative

[akien-mga]

As mentioned in GH-6652.

If you prefer it lowercase, an alternative is to make everything lowercase (including pugixml_FOUND in CMakeLists.txt - and maybe Findpugixml.cmake, not sure).

[Tilka]

All official references to pugixml I can find are lowercase, including the name in its pkgconfig/pugixml.pc file.

[akien-mga]

All official references to pugixml I can find are lowercase, including the name in its pkgconfig/pugixml.pc file.

That's right. The most common usage I've seen in CMake find modules is to use PascalCase or try to follow the upstream "pretty" name, which often has a case different to its library/headers, e.g. FindPhysFS.cmake (with find_package(PhysFS) and PhysFS_FOUND).

But here you're correct that pugixml is always written lowercase, even on upstream's website and its pkgconfig file, so I'll switch everything to lowercase.

[akien-mga]

Updated to all lowercase (including Findpugixml.cmake).