list_pretended: Change how the package name is parsed from stdin

list_pretended: Change how the package name is parsed from stdin

Implement a better way of getting the package name and version from
stdin. This doesn't involve going through a list of all packages in
such a tedious way it was before. Only thing this now requires is that
the version number of any package always starts with a digit. So a
version number like "-1.0" is totally fine, but a version number like
"-r1-1.0" is not. Hopefully the latter ones aren't that common.