Add warnings for private access on traits

[milesziemer]

Previously, it was possible to refer to a trait marked with @private from another namespace. This commit makes doing this cause a warning. A warning is used instead of an error to maintain backward compatibility, but it will be upgraded to an error in the future.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[milesziemer]

I want to try adding a check to see if the invalid trait relationship is via a mixin. That way we can suggest adding localTraits on the mixin.

[milesziemer]

Take this model for example:

namespace example.private

// Mixin with the private trait
@private
@mixin
structure PrivateMixin {}

// Trait uses mixin, inheriting the private trait
@trait
structure someTrait with [PrivateMixin] {}

namespace example.main

use example.private#someTrait

// Using the trait from another namespace
@someTrait
structure ExampleStructure {}

The warning message this PR would produce is:

[WARNING] example.main#ExampleStructure: This shape has an invalid trait relationship that targets a private shape, `example.private#someTrait`, in another namespace.

The issue I see here is that it isn't clear how someTrait is getting the @private trait. In a simple example like this, it's not a big deal, but in a more complex model it could be much more confusing.

Do we want to try to handle this case in this PR, or open a separate issue?

After discussion offline, this issue is out of scope for this PR, so it has been moved out of draft.