Correctly handle multiple interface inheritance

[skinny85]

When faced with an interface inheritance hierarchy like this:

interface CommonActionProps {
  runOrder: number;
  // ...
}

interface CommonActionConstructProps extends CommonActionProps {
  // ...
}

interface CommonPipelineSourceActionProps extends CommonActionProps {
  // ...
}

interface PipelineSourceActionProps extends CommonPipelineSourceActionProps,
    CommonActionConstructProps {
  // ...
}

which is valid TypeScript, JSII barfs when generating PipelineSourceActionProps with:

[ERROR] COMPILATION ERROR : 
[INFO] -------------------------------------------------------------
[ERROR] /tmp/jsii-pacmak-code9apQBY/src/main/java/software/amazon/awscdk/services/s3/PipelineSourceActionProps.java:[37,34] variable _runOrder is already defined in class software.amazon.awscdk.services.s3.PipelineSourceActionProps.Builder
[ERROR] /tmp/jsii-pacmak-code9apQBY/src/main/java/software/amazon/awscdk/services/s3/PipelineSourceActionProps.java:[101,24] method withRunOrder(java.lang.Number) is already defined in class software.amazon.awscdk.services.s3.PipelineSourceActionProps.Builder
[ERROR] /tmp/jsii-pacmak-code9apQBY/src/main/java/software/amazon/awscdk/services/s3/PipelineSourceActionProps.java:[122,42] variable $runOrder is already defined in class <anonymous software.amazon.awscdk.services.s3.PipelineSourceActionProps$Builder$1>
[ERROR] /tmp/jsii-pacmak-code9apQBY/src/main/java/software/amazon/awscdk/services/s3/PipelineSourceActionProps.java:[185,41] method getRunOrder() is already defined in class <anonymous software.amazon.awscdk.services.s3.PipelineSourceActionProps$Builder$1>
[ERROR] /tmp/jsii-pacmak-code9apQBY/src/main/java/software/amazon/awscdk/services/s3/PipelineSourceActionProps.java:[190,29] method setRunOrder(java.lang.Number) is already defined in class <anonymous software.amazon.awscdk.services.s3.PipelineSourceActionProps$Builder$1>
[INFO] 5 errors 
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 3.309 s
[INFO] Finished at: 2018-09-25T23:21:40+00:00
[INFO] Final Memory: 17M/291M
[INFO] ------------------------------------------------------------------------

It obviously tries to include the fields from the CommonActionProps interface twice, because it appears twice in the inheritance hierarchy for PipelineSourceActionProps.

The generators should detect when this happens, and only include the interface in the hierarchy once.

Found in #776.