Import package's modules automatically when they are used as if they were globals

After #43, make the package loader add an `__index` metamethod to `_ENV` to make modules be used like `my_package.submodule.foo()` instead of `local submodule = require("my_package.submodule"); submodule.foo()`

To decide if this should only work with packages or with any kind of module, and also if it should use local packages directly, like if we are in `my_package.module_a`, then `module_b` would behave the same as `my_package.module_b`