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!