Improve CMake options handling

[rdumusc]

• Move all the common_find_package() calls to the top-level CMakeLists. This is needed so that the return() from the common_find_package macro works correctly if a REQUIRED dependency is not found when Brayns is a subproject.
• Replace all

option(BRAYNS_ENABLE_FOO "Enable Foo support" OFF)
if(BRAYNS_ENABLE_FOO)
  common_find_package(foo)
endif()

with:

common_find_package(foo)
if(FOO_FOUND)
  option(BRAYNS_ENABLE_FOO "Enable Foo support" ON)
endif()

This logic has the advantage of enabling all the options that are found by default and still lets users disable them if need be.