rewrite demandload module to be more flexible.
performance not affected.
old demandload module could only import top-level modules like os and
sys.
with rewrite, all of following syntax is emulated perfectly:
import foo, bar foo bar
import foo.bar.quux foo.bar.quux
from foo.bar import quux, blurfl foo.bar:quux,blurfl
only thing missing is "import foo as bar".
also, namespace clashes between placeholder object and demand loaded
module are no longer possible.