packages: close package values on import?

[myitcv]

The package value that results from an import is currently not closed. #629 (comment) and related discussions make the point that such an imported value cannot be made more complete/specific/etc by the importer, save for any approach like cue.mod/usr or aspect oriented package-level augmentations in the main module. But even with such augmentations, it's arguably impossible for the importer to do anything further at the point of importing. As such, closing such a package value helps to shift left errors where references to non-existent fields are caught early.

Capturing this as a separate issue, so that we don't have to point people to a comment buried within an ancient issue.