(REF) CRM_Core_Resources - Limit visibility of `getEntityRefMetadata`

(REF) CRM_Core_Resources - Limit visibility of `getEntityRefMetadata`

Before
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* `CRM_Core_Resources::getEntityRefMetadata()` is nominally `public`.
* The only callers are within `CRM_Core_Resources` and `CRM_Core_ResourcesTest`.
* It seems likely that the method was `public` to support that unit-test.
* This seems like a weird place for the method to live.

After
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* `CRM_Core_Resources::getEntityRefMetadata()` no longer `public`.
* The method is still in a weird place, but at least no one will expect it to remain there.