Fall back to datetime localization of root. when using SFC

[ChristianStornowski]

vue & vue-i18n version

ex: 2.6.10, 8.14.0

Steps to reproduce

1. Create a SFC with <i18n>{"de": {"clockPostfix": "Uhr"}}</i18n> tag and an template like <template><div>{{ $d(new Date(), 'long', 'de') }} {{ $t('clockPostfix') }}</div></template>.
2. Setup i18n config with dateTimeFormate:

   
   const dateTimeFormats = {
   'de': {
       long: {
           day: '2-digit', month: '2-digit', year: 'numeric', minute: '2-digit', hour: '2-digit'
       }
   }
   };

export default new VueI18n({ ... dateTimeFormats })

new Vue({ i18n, ... });

3. Include new created SFC into an app.
4. Start app.

### What is Expected?
No console warning.

### What is actually happening?

[vue-i18n] Fall back to 'de' datetime formats from 'de' datetime formats. [vue-i18n] Fall back to datetime localization of root: key 'long'.

[rdhainaut]

I have the same issue. In waiting, this is a simple workaround that desactive fallback warning:

const i18n = new VueI18n({
  // skip other options config for briefty
  silentFallbackWarn: true
});

[SinBirb]

I would consider this a bug, not an improvement, because there is no way to solve this issue other than silence the warning.