Duplicate class LambdaExpressionCollector

[MaartenS11]

The class proguard.backport.LambdaExpressionCollector is present in both proguard and proguard-core with a slight difference in API. The one in proguard, which I assume is the older version uses Map<Integer, LambdaExpression> lambdaExpressions while the proguard-core version uses Map<InvokeDynamicConstant, LambdaExpression> lambdaExpressions as an argument to the constructor. I assume this class was only part of proguard at one point but was later moved into proguard-core because it could be useful for other projects to work with lambdas but the original class (and some related classes probably) where never removed from the proguard repository resulting in 2 classes with the same name in the same package. We could probably just eliminate this class from proguard and just use the proguard-core class where needed, this would probably require very minor adjustments.

[SwingsOberon]

Hello Maarten Nice to see your name pop up. You are right that this class is duplicated and the changes you propose seem the right way forward. Can you open a PR and request review for it? You can assign me to do so.