handle only deleted files in a <language> - pullrequest build

[scbedd]

In action

I think the only real discussion item here is whether to fold the deleted files directly under ChangedFiles. I prefer it to be separate, as we can rely on ChangedFiles actually resolving to a reference on disk. However for this usage it doesn't actually matter, as we don't rely on them existing on disk to check which package they belong to in Get-PrPackageProperties.

Pending feedback from @weshaggard or @benbp , I have an opinion but I know ya'll will have one as well 👍

@kristapratico when I merge this (regardless of implementation detail) this PR will resolve the issue you saw over here.

[azure-sdk]

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