Fix package.loaded and other cleanups

The listing of the standard modules in package.loaded seems to have been
removed to avoid leaking information to loaded modules. However, since
the *entire* environment is cloned, *including* package.loaded itself,
this does not seem to actually be a problem. But for good measure, also
add a unit test to verify that the version of the standard module tables
referenced from package.loaded is the same as that in _G.

This change also cleans up some unused local variables and an unused
local function from the package module.

Change-Id: I7ec8227b3273059e8f65ad735c215bfd0c623e64