Moving Value interface to the appropriate file

duration.go

diff --git a/duration.go b/duration.go
index db59463..db8bfd5 100644
--- a/duration.go
+++ b/duration.go
@@ -18,13 +18,6 @@
 
 func (d *durationValue) String() string { return (*time.Duration)(d).String() }
 
-// Value is the interface to the dynamic value stored in a flag.
-// (The default value is represented as a string.)
-type Value interface {
-	String() string
-	Set(string) error
-}
-
 // DurationVar defines a time.Duration flag with specified name, default value, and usage string.
 // The argument p points to a time.Duration variable in which to store the value of the flag.
 func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {

flag.go

diff --git a/flag.go b/flag.go
index 9a16e7a..dfc03d2 100644
--- a/flag.go
+++ b/flag.go
@@ -149,6 +149,13 @@
 	Changed   bool   // If the user set the value (or if left to default)
 }
 
+// Value is the interface to the dynamic value stored in a flag.
+// (The default value is represented as a string.)
+type Value interface {
+	String() string
+	Set(string) error
+}
+
 // sortFlags returns the flags as a slice in lexicographical sorted order.
 func sortFlags(flags map[string]*Flag) []*Flag {
 	list := make(sort.StringSlice, len(flags))