This adds support for requesting a minimum Go version instead of an
exact one. If USES contains something like go:X.Y+, we walk the list
of valid Go versions backward and pick the first element that matches
either the requested version or the default version. Assuming the
list of valid versions is sorted semantically, this means we will
pick the requested version if it is newer than the default version,
and the default version otherwise.
This is somewhat imprecise, but it's hard to do better without a
comparison operator that understands semantic versioning, which bmake
lacks. Simply comparing versions lexicographically or numerically
would produce incorrect results, since e.g. 1.3 precedes 1.29 but is
both lexicographically and numerically larger.
Note that specifying a version that does not yet exist in the ports tree
will have the same effect as specifying a version that has been retired:
silently fall back to the default version.
This also drops support for pinning to -devel. This is currently unused
and doesn't work as implemented; furthermore, the current -devel port is
wildly out of date and is expected to be removed rather than updated.