Consider moving tpkg executable logic to library

[jheiss]

Most other ruby command line apps that I've seen have all of their executable logic in a library file. The actual executable file is then usually extremely short, just "require"ing one or two library files. I haven't looked much yet to see if there's an obvious reason why they structure things this way. Possibilities that occur to me might be easier unit testing and/or allowing users to repurpose CLI functionality in their own applications. We should investigate this further and decide if this approach should be taken for the tpkg executable.

Was: https://sourceforge.net/apps/trac/tpkg/ticket/46