[cmake] Move options, correct install include path. fix libxml2-config.cmake

[JackBoosY]

• Move the options for dependencies to the top of find_package
  Reason: Automatically setting options for dependencies based on the search results of dependencies is an inappropriate thing: it may cause actual results to be inconsistent with expectations. So when the option is set to ON but the corresponding dependency cannot be found, it should report the error "Dependency not found".

• Provide macro LIBXML2_INCLUDE_DIR
  Reason: Since libxml2 did not provide cmake export function before, most libraries use FindLibxml2.cmake in cmake, and some libraries use the LIBXML2_INCLUDE_DIR macro provided by cmake. Add this macro to better adapt to existing scenes.

• Use the attributes of the target LibXml2::LibXml2 in libxml2-config.cmake instead of manually setting some macros (such as LIBXML2_INCLUDE_DIRS and LIBXML2_LIBRARIES).
  Reason: Since libxml2 now provides cmake building methods, we should trust the attributes automatically exported by cmake rather than manually implement them. In this way, when we modify the code in CMakeLists.txt, we do not need to modify libxml2.config.cmake.cmake.in synchronously.

Related: microsoft/vcpkg#14588

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