Cleanup source organisation, and make 'gost' an actual module

GOST_CORE_SOURCE_FILES and GOST_ENGINE_SOURCE_FILES were a bit
disorganised, they are now re-arranged so GOST_ENGINE_SOURCE_FILES
contains ENGINE specific source only, and what was less ENGINE
specific was moved to GOST_CORE_SOURCE_FILES.  Furthermore,
GOST_LIB_SOURCE_FILES now includes GOST_CORE_SOURCE_FILES, so the
gost_core library is complete with all implementations.

As a consequence, 'gost' is now explicitly made into a dlopenable
module.  On some operating systems, that makes a difference.

This paves the way for alternative implementations based on the same
base code, such as a provider implementation.

It's quite possible that the re-arrangement done here isn't "pure"
enough.  Future development will tell.