Update get_package_version() to include the short milestone tags.
Update get_package_version() to include the short milestone tags.

Historically, our `get_package_version()` method has returned
alpha/beta versions as, say, `3.0.21alpha1` or `3.0.21beta1`. However,
Python packages use `a` instead of `alpha` and `b` instead of `beta`,
which leads to a warning every time we build a package. Worse, that
difference means we can't reliably call this function to get a suitable
verison string for matching a package filename.

This change updates this method to use the shortened `a` and `b` tags
instead.

Testing Done:
Built a package and saw the correct filename, without warnings.

Used this in an upcoming change to correctly match a generated package
filename.

Reviewed at https://reviews.reviewboard.org/r/11475/

reviewboard/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,chipx86/reviewboard,chipx86/reviewboard,reviewboard/reviewboard,chipx86/reviewboard,reviewboard/reviewboard