Fix replacing shapes when traits applied to mixed-in members

[srchase]

This PR fixes how mixin shapes are updated in the ReplaceShapes transform, which is used by other transforms, such as the excludeShapesByTag or excludeTraitsByTag.

Take this model:

$version: "2.0"

namespace ns.example

@trait()
@tags(["internal"])
string mytrait

@mixin
structure MyMixin {
   foo: String
}

structure StructForMixin with [MyMixin] {
    bar: String
}

apply StructForMixin$foo mytrait("baz")

With this smithy-build.json:

{
  "version": "1.0",
  "projections": {
    "mytransform": {
      "transforms": [
        {
          "name": "excludeTraitsByTag",
          "args": {
            "tags": ["internal"]
          }
        }
      ]
    }
  }
}

Currently, when traits are applied to mixed-in members via an apply statement, a spurious CycleDetection error is thrown when attempting to update the mixed inns.example#StructForMixin$foo member to remove the mytrait trait that was added through an apply statement.

Projection mytransform failed: software.amazon.smithy.model.loader.TopologicalShapeSort$CycleException: Mixin cycles detected among [ns.example#StructForMixin$foo]
software.amazon.smithy.model.loader.TopologicalShapeSort.dequeueSortedShapes(TopologicalShapeSort.java:104)
software.amazon.smithy.model.transform.ReplaceShapes.updateMixins(ReplaceShapes.java:195)

This change prevents extra forward references from being enqueued by the Topological shape sorter that's used to update mixins.

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