dotnetup: Consider refactoring of release info provider APIs

[dsplaisted]

Currently, the release info provider can return the latest ReleaseVersion in a channel. However, to install that version, the DotnetArchiveDownloader has to call ReleaseManifest.FindReleaseFile to get the download URL and hash.

It might be better if the methods to find a release version returned an object that also included the download URL, hash, and any other information needed to download and install the release.

Relatedly, it looks like we have different classes each creating their own ReleaseManifest instance. This means we may be downloading the same releases.json information multiple times in an operation. We should probably try to share a single ReleaseManifest across all the different clases that need it.

[nagilson]

I think the code refactor moved it down to one releases index.

I like the idea of returning additional metadata such as the url to download.