Phase two of package import. "import a.b.c" and all variants now do the

Phase two of package import.  "import a.b.c" and all variants now do the
right thing.

Still to do:

- Make reload() of a submodule work.

- Performance tweaks -- currently, a submodule that tries to import a
global module *always* searches the package directory first, even if
the global module was already imported.  Not sure how to solve this
one; probably need to record misses per package.

- Documentation!