Make Mvc repo folder layout consistent (#7518)

Directory.Build.props

eng/ProjectReferences.props

src/Azure/Azure.sln

src/Identity/Identity.sln

src/Mvc/Directory.Build.props

...Mvc/Mvc.Abstractions/src/Abstractions/ActionDescriptor.cs

...stractions/src/Abstractions/ActionDescriptorExtensions.cs

...tions/src/Abstractions/ActionDescriptorProviderContext.cs

...ractions/src/Abstractions/ActionInvokerProviderContext.cs

...bstractions/src/Abstractions/IActionDescriptorProvider.cs

src/Mvc/Mvc.Abstractions/src/Abstractions/IActionInvoker.cs

...c.Abstractions/src/Abstractions/IActionInvokerProvider.cs

.../Mvc.Abstractions/src/Abstractions/ParameterDescriptor.cs

...ractions/src/ActionConstraints/ActionConstraintContext.cs

...bstractions/src/ActionConstraints/ActionConstraintItem.cs

.../src/ActionConstraints/ActionConstraintProviderContext.cs

...ractions/src/ActionConstraints/ActionSelectorCandidate.cs

...c.Abstractions/src/ActionConstraints/IActionConstraint.cs

...actions/src/ActionConstraints/IActionConstraintFactory.cs

...ctions/src/ActionConstraints/IActionConstraintMetadata.cs