Revert "makeqstrdata.py: Add support for conditionally defined qstrs."

This reverts commit acb133d1b1a68847bd85c545312c3e221a6f7c0b.

Conditionals will be suported using C preprocessor.
py: Preprocess qstrdefs.h before feeding to makeqstrdata.py.

This is alternative implementation of supporting conditionals in qstrdefs.h,
hard to say if it's much cleaner than munging #ifdef's in Python code...
builtinimport: Elaborate debug output support.
builtinimport: Set __path__ attribute on packages.

Per https://docs.python.org/3.3/reference/import.html , this is the way to
tell module from package: "Specifically, any module that contains a __path__
attribute is considered a package." And it for sure will be needed to
implement relative imports.
py: Implement "from pkg import mod" variant of import.
showbc: Dump LOAD_NULL.
showbs: Dump LOAD_CONST_BYTES.