[lldb/CMake] Separate CMake code for Lua and Python (NFC)

Separate the CMake logic for Lua and Python to clearly distinguish
between code specific to either scripting language and the code shared
by both.

What this patch does is:

 - Move Python specific code into the bindings/python subdirectory.
 - Move the Lua specific code into the bindings/lua subdirectory.
 - Add the _python suffix to Python specific functions/targets.
 - Fix a dependency issue that would check the binding instead of
   whether the scripting language is enabled.

Note that this patch also changes where the bindings are generated,
which might affect downstream projects that check them in.

Differential revision: https://reviews.llvm.org/D85708