rename package_version to get_package_version

Most of the other package_version uses are a variable,
which makes it a bit confusing imho.
Skip updating the version if no changes have been found

The non-Plain-Version Version string that we generate contains
the current timestamp, so any run of the script would generate
a diff in the Version: line, triggering needless rebuilds and
source checkins. Only change the Version line if there was a
code change. In cases when the %setup line or the source tarball
does not match, still do the substitution of those.
Merge pull request #12 from dirkmueller/sticky_timestamps

Sticky timestamps