Currently, the implementation requires returning the absolute path to a resource manifest - but I
can already see use cases for discovering extension manifests. For example, I might have
installed an extension from WinGet, and need to discover that, too.

Right now, it expects JSON like this:

{"resourceManifestPath":"/foo/bar/baz.dsc.resource.json"}

But we could have it return this, and parse the last segment of the path to determine kind:

{"absoluteManifestPath":"/foo/bar/baz.dsc.extension.json"}

Or require it return either resourceManifestPath or extensionManifestPath.

If we support extensions discovering extensions, we would need to recursively handle extension
discovery.

I think we can defer extension discovery til when we need it

Renamed from resourceManifestPath to just manifestPath to reuse extensions in the future. I don't think we need to prefix with absolute