Separates the MMP into two phases:
1). Creation of the ModelMetadata, discovery of properties and attributes
(reflection) is part of the MMP
2). Lookup of details based on attributes is now part of another phase,
and has its results cached.
Users can now implements and register an IFooMetadataProvider to customize
a single aspect of metadata (see how data annotations does it).
- a single test is failing under CoreCLR only
- using the resource-related properties of a `ValidationAttribute` in
precisely one place – the failing test.
- in .NET the DataAnnotations assembly is somehow able to get access to
`internal` resources
- that doesn’t work under CoreCLR
- short-term fix is to add an `[InternalsVisibleTo]` in the test
assembly, giving DataAnnotations access to the resource
Will also file a bug in the DataAnnotations repo