fix: codegen structure members more safely to avoid conflicts with package names

[ianbotsf]

Issue #

(none)

Description of changes

Presently, a structure member which shares a name with a top-level package cannot be codegenerated by smithy-kotlin. For example, if a structure Foo has a member aws and the service's fully-qualified package starts with aws. the following code will be generated (heavily trimmed):

class Foo {
    val aws: Type? = ...
    val otherMember: Type? = ...

    class Builder {
        var aws: Type? = null
        var otherMember: Type? = null

        // DSL builder method
        fun otherMember(block: aws.fully.qualified.package.Type.Builder.() -> Unit) {
            // This line fails to compile because `aws` resolves to the member rather than the top-level namespace.
            // The error will be something like "Unresolved reference 'fully'".
            this.otherMember = aws.fully.qualified.package.Type.invoke(block)
        }
    }
}

Unfortunately, there's no way to unambiguously refer to a package name when its shadowed inside of a statement scope. Since the conflict does not exist at the file level, this PR solves the problem by capturing references to necessary builder objects in file-local variables and invoking those in DSL builder methods instead:

// Capture an unambiguous reference to the companion object
val awsFullyQualifiedPackageTypeDslBuilderRef = aws.fully.qualified.package.Type

class Foo {
    val aws: Type? = ...
    val otherMember: Type? = ...

    class Builder {
        var aws: Type? = null
        var otherMember: Type? = null

        // DSL builder method
        fun otherMember(block: aws.fully.qualified.package.Type.Builder.() -> Unit) {
            // Invoke DSL via the previously captured ref
            this.otherMember = awsFullyQualifiedPackageTypeDslBuilderRef(block)
        }
    }
}

I don't love this and I'm open to alternative approaches.

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

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