checkInputs used to be added to nativeBuildInputs. Now we have
nativeCheckInputs to do that instead. Doing this treewide change allows
to keep hashes identical to before the introduction of
nativeCheckInputs.
This commit splits the Zycore dependency that was previously pulled
in via `fetchSubmodules` into a separate derivation. The package
previously didn't install the Zycore headers and CMake configs, making
it impossible to actually compile any code that uses Zydis because Zydis
includes Zycore headers.
