Development versions of pkg have a version like "2.3.0-d8bbde1c3-dirty",
which is not handled by PKG_BIN_VERSION, so PKG_EXT ends up being "tar"
and this breaks update-packages.

Fix the PKG_BIN_VERSION logic, and while here, remove backwards compat
for pkg versions prior to 2.0, which no one should be using to build
main or stable/15. This means nothing is left to use PKG_BIN_VERSION,
but continue to set it, since we might need it again in the future.

MFC after: 3 days