When using resolvers, use last value only.

This fixes an issue where the value and all its hooks would
be applied for each resolver, making it difficult to compose multiple
config files.

context.go

@@ -444,26 +444,34 @@ func (c *Context) Resolve() error {
 			if _, ok := c.values[flag.Value]; ok {
 				continue
 			}
+
+			// Pick the last resolved value.
+			var selected interface{}
 			for _, resolver := range resolvers {
 				s, err := resolver.Resolve(c, path, flag)
 				if err != nil {
-					return err
+					return errors.Wrap(err, flag.ShortSummary())
 				}
 				if s == nil {
 					continue
 				}
-
-				scan := Scan().PushTyped(s, FlagValueToken)
-				delete(c.values, flag.Value)
-				err = flag.Parse(scan, c.getValue(flag.Value))
-				if err != nil {
-					return err
-				}
-				inserted = append(inserted, &Path{
-					Flag:     flag,
-					Resolved: true,
-				})
+				selected = s
 			}
+
+			if selected == nil {
+				continue
+			}
+
+			scan := Scan().PushTyped(selected, FlagValueToken)
+			delete(c.values, flag.Value)
+			err := flag.Parse(scan, c.getValue(flag.Value))
+			if err != nil {
+				return err
+			}
+			inserted = append(inserted, &Path{
+				Flag:     flag,
+				Resolved: true,
+			})
 		}
 	}
 	c.Path = append(inserted, c.Path...)