small gotcha with Go ports

I was having trouble (re)building devel/git-lfs and traced it to the fact
that the vendored module imports were out of date (so `make distclean` from
the port, or removing the appropriate distfiles `go/` subdirectory, made it
build correctly unattended again - though I failed to save everything first
to see what changed).

This makes me wonder if Mk/Uses/go.mk should auto-clean when the
DISTVERSION (and/or GO_MODULE?) is changed. It's a bit brute-force but it
matches the general behavior of most ports. Or, perhaps just run `go mod
tidy` earlier? We have a `go mod tidy -e` step in go-post-extract, but by
then it's too late. There are a bunch of failures (ignored because of `-x`)
in the `go-post-fetch` step.

I'm not enough of an expert on either the go.mk file or in Go itself to
know where all the pitfalls might be. (Also, with it building automatically
again, I can just ignore the problem for now :-) )

Chris