feat(goctl): supports api multi-level importing (#1747)

* feat(goctl): supports api  multi-level importing

Resolves: #1744

* fix(goctl): import-cycle, etc.

import-cycle will not be allowed
e.g., a.api -> b.api -> a.api
regular multiple-import will be allowed
e.g., a.api -> b.api -> c.api
                   -> c.api

* refactor(goctl): adds comments to exported var

* fix(goctl): typo in a comment

tools/goctl/api/parser/g4/ast/importstack.go

@@ -0,0 +1,23 @@
+package ast
+
+import "errors"
+
+// ErrImportCycleNotAllowed defines an error for circular importing
+var ErrImportCycleNotAllowed = errors.New("import cycle not allowed")
+
+// importStack a stack of import paths
+type importStack []string
+
+func (s *importStack) push(p string) error {
+	for _, x := range *s {
+		if x == p {
+			return ErrImportCycleNotAllowed
+		}
+	}
+	*s = append(*s, p)
+	return nil
+}
+
+func (s *importStack) pop() {
+	*s = (*s)[0 : len(*s)-1]
+}