When using builtin_openssl=ON, CMake erroneously exports the builtin
static libraries in ROOTConfig-targets.cmake, which causes problems with
dependent projects. In order to avoid this, we need Net and RHTTP to use
SSL as a private dependency. Since CMake requires target_link_libraries()
to either be all-plain (as before) or all-keyword (required to allow
PRIVATE linking), we need to move the build system to use all-keyword
linking only.

Fixes: ROOT-9532