Problem: our shallowing of directories is too narrow

[clacke]

https://docs.racket-lang.org/pkg/Package_Concepts.html#%28tech._package._source%29 explicitly says:

An archive represents package content analogous to a directory, but if the archive’s content is contained within a single top-level directory, then the directory’s content (as opposed to the overall archive content) is used as the package content.

So our current shallowing routine, only shallowing a directory with the same name as the unpacked file, is too narrow. We could just go and shallow every source directory that has a single directory in its root. Take care not to miss any hidden files.