6497: Separate package build and uploader scripts.

6497: Separate package build and uploader scripts.

This lets us avoid redundant uploads as we build for multiple targets,
and lets us better customize the upload process per target.

run-build-packages had code to avoid doing an upload when there was no
new package to build.  run_upload_packages uses a timestamp file to
achieve a similar result.

Other changes to run-build-packages to support this:

* Better detection of when Python and Ruby packages have already been
  built.
* Refactor Gem building into a function to fix various bugs (wrong
  version checked for existing arvados-cli; distro packages not
  installed in the proper directory).
* Refactor git log querying to reduce code duplication.