[errors] Greatly simplify common/errors package.

common/config/validation/validation.go

 // Copyright 2017 The LUCI Authors. All rights reserved.
 // Use of this source code is governed under the Apache License, Version 2.0
 // that can be found in the LICENSE file.
 
 // Package validation provides a helper for performing config validations.
 package validation
 
 import (
         "fmt"
         "strings"
@@ skipping 28 matching lines @@
 type Context struct {
         Logger logging.Logger // logs errors as they appear
 
         errors  errors.MultiError // all accumulated errors
         file    string            // the currently validated file
         element []string          // logical path of a sub-element we validate, see Enter
 }
 
 type fileTagType struct{ Key errors.TagKey }
 
-func (f fileTagType) With(name string) errors.TagValue { return errors.MkTagValue(f.Key, name) }
+func (f fileTagType) With(name string) errors.TagValue {
+        return errors.TagValue{Key: f.Key, Value: name}
+}
 func (f fileTagType) In(err error) (v string, ok bool) {
         d, ok := errors.TagValueIn(f.Key, err)
         if ok {
                 v = d.(string)
         }
         return
 }
 
 type elementTagType struct{ Key errors.TagKey }
 
 func (e elementTagType) With(elements []string) errors.TagValue {
-        return errors.MkTagValue(e.Key, append([]string(nil), elements...))
+        return errors.TagValue{Key: e.Key, Value: append([]string(nil), elements...)}
 }
 func (e elementTagType) In(err error) (v []string, ok bool) {
         d, ok := errors.TagValueIn(e.Key, err)
         if ok {
                 v = d.([]string)
         }
         return
 }
 
 var fileTag = fileTagType{errors.NewTagKey("holds the file name for tests")}
@@ skipping 61 matching lines @@
 //
 // Returns nil otherwise.
 func (v *Context) Finalize() error {
         if len(v.errors) == 0 {
                 return nil
         }
         return &Error{
                 Errors: append(errors.MultiError{}, v.errors...),
         }
 }