Perform Repository Trust Check at Install, not Search, Time

[anamnavi]

PR Summary

Currently when installing without specifying a repository, PSGet prompts the user for each untrusted repository while searching- even if that repository would not contain the package. This PR moves this check to happen at install time so the user only has to approve the repository the package would actually be downloaded from and minimalizes prompting to where necessary.

PR Context

Resolves #795

PR Checklist

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