Revert "pkgbase: differentiate package versions for ALPHA/BETA/PRERELEASE/RC phases"

This reverts commit aef769614f921660cb0262412426034cf5395ae5.

While the package names are a bit prettier this confuse pkg about upgrading :
$ pkg version -t 13.0.s2021011313063 13.0.a1

$ pkg version -t 13.0.s2021011313063 13.0_ALPHA1
<

Note that the current scheme isn't good when bumping from ALPHA to BETA or
even BETA to RC:
$ pkg version -t 13.0_ALPHA1 13.0_BETA1

$ pkg version -t 13.0_BETA1 13.0_RC1

But more thoughts have to be put into this renaming.