Run (go vet) on all subpackages instead only of changed files

Apparently, it was never documented to use (go vet $somefile.go)
(but (go tool vet $somefile.go) was).

go 1.10 seems to do more checks within packages, and $somefile.go
is interpreted as a package with only that file (even if other files
from that package are in the same directory), leading to spurious
"undefined: $symbol" errors.

So, just run (go vet) on ./... (explicitly excluding skopeo/vendor for the
benefit of Go 1.8). We only have three subpackages, so the savings, if any,
from running (go vet) only on the modified subpackages would be small.

More importantly, on a toolchain update, ./... allows us to see the newly
detected issues all at once, instead of randomly waiting for a commit that
changes one of the affected files for the failure to show up.

hack/make/validate-vet

@@ -1,28 +1,13 @@
 #!/bin/bash
 
-source "$(dirname "$BASH_SOURCE")/.validate"
+errors=$(go vet $(go list -e ./... | grep -v "$SKOPEO_PKG"/vendor))
 
-IFS=$'\n'
-files=( $(validate_diff --diff-filter=ACMR --name-only -- '*.go' | grep -v '^vendor/' || true) )
-unset IFS
-
-errors=()
-for f in "${files[@]}"; do
-	failedVet=$(go vet "$f")
-	if [ "$failedVet" ]; then
-		errors+=( "$failedVet" )
-	fi
-done
-
-
-if [ ${#errors[@]} -eq 0 ]; then
+if [ -z "$errors" ]; then
 	echo 'Congratulations!  All Go source files have been vetted.'
 else
 	{
 		echo "Errors from go vet:"
-		for err in "${errors[@]}"; do
-			echo " - $err"
-		done
+		echo "$errors"
 		echo
 		echo 'Please fix the above errors. You can test via "go vet" and commit the result.'
 		echo