Fix bug where mixin cycles were incorrectly detected

[hpmellema]

Description of bug
A bug was discovered where mixin cycles would be incorrectly detected if a non structure or union shape had a trait that was removed by a transform.

For example:
The following smithy model

// main.smithy
namespace example

@trait
@tags(["filtered"])
structure myTrait {}

@myTrait
operation OperationWithMixin with [ OperationMixin ] {
    input := {
        @required
        myInputField: String,
        other: String
    }
}

Using the following Transform:

{
  "version": "2.0",
  "projections": {
    "myProjection": {
      "transforms": [
       {
          "name": "excludeTraitsByTag",
          "args": {
            "tags": ["filtered"]
          }
        },
    ]}
  }
}

Would encounter a build exception like

Projection myProjection failed: software.amazon.smithy.model.loader.TopologicalShapeSort$CycleException: Mixin cycles detected among [example#OperationWithMixin]
software.amazon.smithy.model.loader.TopologicalShapeSort$CycleException: Mixin cycles detected among [example.#OperationWithMixin]

---

Description of changes:

• Adds new public methods to the Model class to access shapes using mixins and mixin shapes
• Uses the above methods to add all mixins and shapes with mixins to topological sorter in the ReplaceShapes class so that cycle detection can run correctly (previously only unions and structures were added)
• Adds tests for the specific error encountered above
• Adds simple unit test for methods added to Model class.

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