Revert "Use - instead of _ to seperate pak name fields". Technically wrong.

This reverts commit a7ac70fc721e732cdbf97a5f291cf31e149e6abf.

Going for what's technically correct here.

History: _ and - used arbitrarily.

There is precedent for following Debian's versioning methods:
* Sorting method in use (adapted from dpkg).
* Use of ~ for pre-release (1~1 < 1 < 1.1).

Also, - gets used in version numbers (I've not seen _ used); and it makes
sense for us to have it available, e.g.  for use in version nos.  of patched
third-party maps.