Provide a centralized lookup for PluginDownloadURL

[iwahbe]

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Issue details

The registry should be able to serve as a centralized lookup table to go from a plugin name like prov (implicitly pulumi/prov) or myGithubOrg/my-prov to an appropriate plugin download URL. Right now, downloading a provider not hosted in the pulumi org at it's canonical repo (pulumi/pulumi-${PLUGIN_NAME})mrequired passing --plugin-download-url. The registry already has access to all the information necessary to serve this information.

This is effectively a derivative case of #2388.

The pulumi CLI currently maintains a hard-coded list of plugin -> download URL mappings, which does not scale with community packages or allow packages to move (since only the latest version of pulumi is up to date).

Affected area/feature