As described in
https://github.com/yuzu-emu/yuzu/pull/9395#discussion_r1047456172
checking for PKG_CONFIG_FOUND before calling pkg_search_module() is
unneeded, and some find modules (like FindFFmpeg.cmake) don't do this
already. Consequently, this patch removes these checks.

cmake: correct find modules

Uses find_package_handle_standard_args to handle the find_package call
from the root CMakeLists. Removes all the unnecessary logic after the
find_package and just sets it to REQUIRED.

This better matches upstream's FindOpus.cmake file, and it will make
using upstream's FindOpus.cmake file easier.

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.