cmd/go: skip files tagged ignore in go mod vendor #31088
Description
In reviewing https://golang.org/cl/169637, I noticed that one of the larger files added in https://golang.org/cl/164623 is src/vendor/golang.org/x/text/unicode/norm/maketables.go, which is tagged // +build ignore.
Such files are never included as part of the build graph and explicitly ignored by go mod tidy (#29598), so we should filter them out, much as we already filter out test sources and data (see https://golang.org/cl/122256).
CC @jayconrod @mpvl; compare #25873; contrast #26366.