Polish FindBoostPython.cmake from CMake's point of view:

 * Use BoostPython_ prefix instead of BOOST_PYTHON_
   (BOOSTPYTHON_ is also acceptable but is unreadable);

 * Remove extra call to find_package(BoostPython);

 * Replace macro_optional_find_package() with simple find_package(),
   CMake has CMAKE_DISABLE_FIND_PACKAGE_Foo feature for a while.

This is a first patch of a bigger set, containing only minor fixes.

CCBUG: 320807
REVIEW: 110953