Add support for version_file in pkg_tar2rpm

[phlax]

It would be very helpful if a version file was supported for setting the package version - as eg pkg_deb does

if a version file could be passed in it would allow pkg_tar2rpm to read version info directly from eg a VERSION file rather than having to duplicate/hardcode the current version in BUILD/bzl files in addition to the canonical setting

[jarondl]

What is the structure of the VERSION file? are we talking about a text file containing only one string? And do you want to pass it as the version flag to rpmpack? If so, does this only require changes in the bzl file..
https://github.com/google/rpmpack/blob/master/def.bzl#L6

[phlax]

are we talking about a text file containing only one string?

yep, i think that is how pkg_deb works (cf https://docs.bazel.build/versions/1.0.0/be/pkg.html#pkg_deb)