[http-client-csharp] Generator should not override the file it just writes

[ArcturusZhang]

Now we could have classes in different namespaces with the same name, but those files are still written into the same directory, such as Models.
There would be occasions that the generator writes one file A.Foo and then writes B.Foo into the same file Foo.cs.

We should have a mechanism to avoid this, because from the user's perspective, this is very tricky to know where it goes wrong.

For instance, we could write the first class A.Foo into Foo.cs, and then write class B.Foo into Foo.1.cs which should not trigger the linter of C# that the filename should be the same as the first type it defines.
Or we could change the structure to just follow the namespace like java.

[JoshLove-msft]

We would need separate model directories for each namespace. We could just only have the separate directories when there is a conflict as this doesn't cause any breaking changes.

On the other hand, it might be better to just always be consistent. In C# we usually like to keep things flat.

The approach of renaming the file makes sense if we go the route of only changing the behavior when there is a conflict. If we are going to go the directory route, it is probably better to just always do this.