fix(config): tie save-exact/save-prefix together

Each affects the other

lib/utils/config/definitions.js

@@ -1526,7 +1526,10 @@ define('save-exact', {
     Dependencies saved to package.json will be configured with an exact
     version rather than using npm's default semver range operator.
   `,
-  flatten,
+  flatten (key, obj, flatOptions) {
+    // just call the save-prefix flattener, it reads from obj['save-prefix']
+    definitions['save-prefix'].flatten('save-prefix', obj, flatOptions)
+  },
 })
 
 define('save-optional', {
@@ -1595,6 +1598,7 @@ define('save-prefix', {
   `,
   flatten (key, obj, flatOptions) {
     flatOptions.savePrefix = obj['save-exact'] ? '' : obj['save-prefix']
+    obj['save-prefix'] = flatOptions.savePrefix
   },
 })