[feature] CMake generators: include 'build_modules' after creating the packages

[jgsogo]

Duplicated: #7261

---

This is a last-mile problem for CMake. Some packages provide CMake modules with some customization that doesn't fit the general conventions and we cannot keep adding more complexity to the cpp_info model:

• OpenSSL has a different casing for the filename and the variables provided by it: https://cmake.org/cmake/help/v3.6/module/FindOpenSSL.html
• cereal provides a cereal-config.cmake that offers targets without namespace ([package] cereal/1.3.0: cereal cannot be found when using the cmake_paths generator conan-center-index#2151), SPIRV-tools too (IIRC).
• ...there are many details hidden in other packages

If we really want to provide transparent integration, this last-mile details cannot be supported in the cpp_info, but they can be added to the build_modules, those files are included for all the CMake generates and they can populate other variables based on the value of existing ones and they could create custom targets (or alias)...

To be able to implement this feature, some work/research to be done:

• include build_modules after creating the targets for cmake_find_package[_multi] generators.
• build_modules need to know the generator used, so they know if targets are available or not (depending on the generator, or even depending on the TARGET argument to conan_basic_setup).

[jgsogo]

Closing as duplicate #7261