ResolvedDependencyKey should be struct to reduce memory allocations

[jeffkl]

The NuGet.Commands.ResolvedDependencyKey class is allocated a lot during dependency resolution and instead could be a struct which would reduce memory usage. This would be a breaking change for any user who is using the API. This is a list of the public APIs that will change:

NuGet.Commands.ResolvedDependencyKey.Equals(NuGet.Commands.ResolvedDependencyKey other) -> bool
NuGet.Commands.ResolvedDependencyKey.ResolvedDependencyKey() -> void
static NuGet.Commands.ResolvedDependencyKey.operator !=(NuGet.Commands.ResolvedDependencyKey left, NuGet.Commands.ResolvedDependencyKey right) -> bool
static NuGet.Commands.ResolvedDependencyKey.operator ==(NuGet.Commands.ResolvedDependencyKey left, NuGet.Commands.ResolvedDependencyKey right) -> bool

Also, the NuGet.Commands.UnexpectedDependencyMessages was removed from the public API