Consistent annotation names

Add `Annotation` suffix to the private annotations to allow nicer code
using the constants.

For example one can use the current annotation names as a temporary
variable instead of unclear shortcut. Instead of this:

    rag := flagsFromAnnotation(c, f, requiredAsGroup)
    me := flagsFromAnnotation(c, f, mutuallyExclusive)
    or := flagsFromAnnotation(c, f, oneRequired)

We can use now:

    requiredAsGrop := flagsFromAnnotation(c, f, requiredAsGroupAnnotation)
    mutuallyExclusive := flagsFromAnnotation(c, f, mutuallyExclusiveAnnotation)
    oneRequired := flagsFromAnnotation(c, f, oneRequiredAnnotation)

Example taken from spf13#2105.

flag_groups.go

@@ -23,9 +23,9 @@ import (
 )
 
 const (
-	requiredAsGroup   = "cobra_annotation_required_if_others_set"
-	oneRequired       = "cobra_annotation_one_required"
-	mutuallyExclusive = "cobra_annotation_mutually_exclusive"
+	requiredAsGroupAnnotation   = "cobra_annotation_required_if_others_set"
+	oneRequiredAnnotation       = "cobra_annotation_one_required"
+	mutuallyExclusiveAnnotation = "cobra_annotation_mutually_exclusive"
 )
 
 // MarkFlagsRequiredTogether marks the given flags with annotations so that Cobra errors
@@ -37,7 +37,8 @@ func (c *Command) MarkFlagsRequiredTogether(flagNames ...string) {
 		if f == nil {
 			panic(fmt.Sprintf("Failed to find flag %q and mark it as being required in a flag group", v))
 		}
-		if err := c.Flags().SetAnnotation(v, requiredAsGroup, append(f.Annotations[requiredAsGroup], strings.Join(flagNames, " "))); err != nil {
+		values := append(f.Annotations[requiredAsGroupAnnotation], strings.Join(flagNames, " "))
+		if err := c.Flags().SetAnnotation(v, requiredAsGroupAnnotation, values); err != nil {
 			// Only errs if the flag isn't found.
 			panic(err)
 		}
@@ -53,7 +54,8 @@ func (c *Command) MarkFlagsOneRequired(flagNames ...string) {
 		if f == nil {
 			panic(fmt.Sprintf("Failed to find flag %q and mark it as being in a one-required flag group", v))
 		}
-		if err := c.Flags().SetAnnotation(v, oneRequired, append(f.Annotations[oneRequired], strings.Join(flagNames, " "))); err != nil {
+		values := append(f.Annotations[oneRequiredAnnotation], strings.Join(flagNames, " "))
+		if err := c.Flags().SetAnnotation(v, oneRequiredAnnotation, values); err != nil {
 			// Only errs if the flag isn't found.
 			panic(err)
 		}
@@ -70,7 +72,8 @@ func (c *Command) MarkFlagsMutuallyExclusive(flagNames ...string) {
 			panic(fmt.Sprintf("Failed to find flag %q and mark it as being in a mutually exclusive flag group", v))
 		}
 		// Each time this is called is a single new entry; this allows it to be a member of multiple groups if needed.
-		if err := c.Flags().SetAnnotation(v, mutuallyExclusive, append(f.Annotations[mutuallyExclusive], strings.Join(flagNames, " "))); err != nil {
+		values := append(f.Annotations[mutuallyExclusiveAnnotation], strings.Join(flagNames, " "))
+		if err := c.Flags().SetAnnotation(v, mutuallyExclusiveAnnotation, values); err != nil {
 			panic(err)
 		}
 	}
@@ -91,9 +94,9 @@ func (c *Command) ValidateFlagGroups() error {
 	oneRequiredGroupStatus := map[string]map[string]bool{}
 	mutuallyExclusiveGroupStatus := map[string]map[string]bool{}
 	flags.VisitAll(func(pflag *flag.Flag) {
-		processFlagForGroupAnnotation(flags, pflag, requiredAsGroup, groupStatus)
-		processFlagForGroupAnnotation(flags, pflag, oneRequired, oneRequiredGroupStatus)
-		processFlagForGroupAnnotation(flags, pflag, mutuallyExclusive, mutuallyExclusiveGroupStatus)
+		processFlagForGroupAnnotation(flags, pflag, requiredAsGroupAnnotation, groupStatus)
+		processFlagForGroupAnnotation(flags, pflag, oneRequiredAnnotation, oneRequiredGroupStatus)
+		processFlagForGroupAnnotation(flags, pflag, mutuallyExclusiveAnnotation, mutuallyExclusiveGroupStatus)
 	})
 
 	if err := validateRequiredFlagGroups(groupStatus); err != nil {
@@ -232,9 +235,9 @@ func (c *Command) enforceFlagGroupsForCompletion() {
 	oneRequiredGroupStatus := map[string]map[string]bool{}
 	mutuallyExclusiveGroupStatus := map[string]map[string]bool{}
 	c.Flags().VisitAll(func(pflag *flag.Flag) {
-		processFlagForGroupAnnotation(flags, pflag, requiredAsGroup, groupStatus)
-		processFlagForGroupAnnotation(flags, pflag, oneRequired, oneRequiredGroupStatus)
-		processFlagForGroupAnnotation(flags, pflag, mutuallyExclusive, mutuallyExclusiveGroupStatus)
+		processFlagForGroupAnnotation(flags, pflag, requiredAsGroupAnnotation, groupStatus)
+		processFlagForGroupAnnotation(flags, pflag, oneRequiredAnnotation, oneRequiredGroupStatus)
+		processFlagForGroupAnnotation(flags, pflag, mutuallyExclusiveAnnotation, mutuallyExclusiveGroupStatus)
 	})
 
 	// If a flag that is part of a group is present, we make all the other flags