build: simplify find modules

With this patch I've deleted a few find modules that are now unused
since the vcpkg transition, as the CMake code now forces CONFIG mode for
Catch2, fmt and nlohmann_json.

I've then simplified the lz4, opus, and zstd modules by exclusively
using pkg-config. They were using it already, but were ignoring the
result. Also, I believe that manually looking for libraries was required
for Conan to work, and it is thus not needed anymore.

Lastly, I believe that there is no platform that ships these system libs
without pkg-config/pkgconf, so requiring it should be fine.

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