Provide distinct Usings for final generated compliation unit

[stevenknox]

When using multiple different generators that have an overlap of Using Directives , the resulting consumer project may get a CS0105 warning "The using directive for 'Foo' appeared previously in this namespace"

I have applied Distinct() to ensure there are no duplicates in the final generated compilation.

[AArnott]

Often in Roslyn's OM, the types don't override the Equals method with by-value semantics. Have you confirmed that this change is effective across multiple generators mentioned in your scenario? If so, it looks good.

[amis92]

Please add a test based on

https://github.com/AArnott/CodeGeneration.Roslyn/blob/fd1200769415b5428fbd6c5859dbe55cf78f0b7b/src/CodeGeneration.Roslyn.Tests.Generators/AddGeneratedUsingGenerator.cs

and

https://github.com/AArnott/CodeGeneration.Roslyn/blob/master/src/CodeGeneration.Roslyn.Tests/DocumentTransformTests.cs

Shouldn't be too complicated :)

[stevenknox]

Perfect! ill get a test added for this first thing in the morning =)

[AArnott]

This project is getting archived in favor of Source Generators. See #229