[portgen go] try harder to determine the version

Hi,

Recently a program was found that caused breakage in 'portgen go'. The
breakage was two fold:

1) https://proxy.golang.org/qvl.io/promplot/@latest returns unexpected
results. This caused portgen to bomb out.
2) Even it 1) had worked, the logic in 'get_ver_info' was broken and it
picked the wrong version.

This diff changes things so we first try to get our version from
'/@latest'. If that fails, we call `get_ver_info` which pulls down the
full list of versions from the '/@v/list' endpoint.

Thanks to afresh1 for showing me the neat '//=' perl trick!

Tested with:
portgen go qvl.io/promplot

as well as a number of other ports.

OK? Cluesticks?