cmd/vet: move jsontags into vet (#17777)

The cmd/jsontags is non-idiomatic since it is not a main binary.
Move it to a vet directory, which will eventually contain a vettool binary.

Update tailscale/corp#791

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

cmd/vet/jsontags/report.go

@@ -28,9 +28,9 @@ var jsontagsAllowlist map[ReportKind]set.Set[string]
 //
 // The struct type name may be "*" for anonymous struct types such
 // as those declared within a function or as a type literal in a variable.
-func ParseAllowlist(b []byte) map[ReportKind]set.Set[string] {
+func ParseAllowlist(s string) map[ReportKind]set.Set[string] {
 	var allowlist map[ReportKind]set.Set[string]
-	for line := range strings.SplitSeq(string(b), "\n") {
+	for line := range strings.SplitSeq(s, "\n") {
 		kind, field, _ := strings.Cut(strings.TrimSpace(line), "\t")
 		if allowlist == nil {
 			allowlist = make(map[ReportKind]set.Set[string])