elf2efi: import whole module, not individual symbols

elf2efi: import whole module, not individual symbols

When reading the code, it was hard to figure out if the given name was
imported or a local class. And the renaming of imports also made it
harder to look things up online. Arguably, the deeply nested import
structure and inconsistent naming in elftools is partially to blame:
there is just no good way to make this look nice. But anyway, let's use
the usual style of importing the module and using names prefixed with
the module path so that the origin of imported names is clear.

elfutils.elf.elffile is importered separately, because a) it needs to be
imported separately anyway bxecause the module does lazy imports
internally, a) the name already indicates the origin, c) is used in
quite a few places so the shorter name is nice.