6388: Improve portability of Python backports in run-build-packages.

6388: Improve portability of Python backports in run-build-packages.

* Prefix package names with the same prefix used by the interpreter
  package, to avoid conflicts with system packages when we're using
  something like Software Collections.
* Always specify a dependency on that package.
* Exclude more test paths from packages to avoid conflicts.
* Do a little name munging to make sure that output package names
  match dependencies generated by fpm.