Fix compilation error in generate code caused by name collision (#3175)

## Motivation and Context
If you had a model like this:
```smithy
   @http(uri: "/SomeOperation2", method: "GET")
            operation GetThing {
                // input: GetThingInput,
                output: GetThingOutput
            }
```

But then nested in some other API you did something like this:
```smithy
            list GetThings {
                member: GetThingOutput
            }
```

Code would fail to compile because we generated the same method
signature for two different types.

## Description
<!--- Describe your changes in detail -->

## Testing
- [x] fixes minimal reproducer

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