Simpler version of pkg_repl()

A preliminary implementation of pkg_repl() to help with discussion in #67... This is to illustrate what could be a simplified pkg_repl() that will retain the main features of existing pkg_repl(), including ability to define package versions in a nlist (package_default).
To help the discussion I submit this proposal before really testing it. It has at least one flaw: you cannot add a second arch for an existing package, this should be easy to fix.
If/when there is a consensus on what should be done, I could rewrite other pkg_xxx functions that are currently broken as they were calling pkg_repl() in a way not supported by the new function.