Skip sources when runtime provider is not found

[nohwnd]

We don't unify to a single framework anymore, so frameworks like .NET Portable don't get unified to .NET Framework anymore. So we need to rather skip unknown runtime providers than fail, because before multi-tfm it just unified to .NET Framework and passed, or unified to .NET "core" and failed.

This could be fixed by simply changing the inferred framework, but instead it is better to have a way to explicitly skip a source, instead of running it on testhost that is hopefully compatible and does nothing, or in the future also have an option to explicitly fail if we ever introduce a strict mode.

Fix #3737

[nohwnd]

This is what happens when we force a framework to be net472:

This is what happens if we don't force a framework:

Before the change the second scenario would fail, because it could not find runtime provider for .NET Portable. This was a breaking change from non-multitfm.

[nohwnd]

@shyamnamboodiripad we are changing the partial discovery api to keep it extensible and to indicate sources that were knowingly skipped. We plan to later add aditional fields to the DiscoveredSource object, so we can tell TE why that particular source failed / skipped.

This needs us to coordinate insertion into TE, because we are changing api that is published, but not released yet. I will prepare the change for TE tomorrow.

[nohwnd]

/azp run

[azure-pipelines]

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