cmd/go/internal/modcmd: skip files with the "ignore" constraint in 'go mod vendor'

'go mod vendor' already drops test files and testdata directories, so
users should not expect the vendored module to include unnecessary
files in general.

Files tagged "ignore" are typically only used to refresh or regenerate
source files within the module to be vendored, so users of that module
do not need them.

Fixes #31088

Change-Id: I1ce9545e9b37c8e779a1826a9d494ac29d2cbfb8
Reviewed-on: https://go-review.googlesource.com/c/go/+/172978


Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>