Convert IEnumerables to Lists for perf improvements

[alerickson]

PR Summary

Pervasive use of IEnumerables in primarily FindHelper and InstallHelper were causing some unexpected performances degradations. Occasionally the same call was (unexpectedly) occurring multiple times due to the fact that IEnumerable does not return a collection. This was particularly painful because it meant multiple, unnecessary server calls, which can hit the server hard, but also decreases performance of PowerShellGet.

PR Context

Resolves #654:

• LINQ is generics and can cause multiple compiled methods calling search APIs.
• Find code always returned IEnumerable interface, causing LINQ calls to call search APIs multiple times to manipulate IEnumerable as collections.
• Dependency search returns IEnumerable which is then used as collection and calls server X times.
  retrievedPkgs = pkgMetadataResource.GetMetadataAsync(...)
  if (retrievedPkgs == null || retrievedPkgs.Count() == 0) ...
  foundPackagesMetadata.AddRange(retrievedPkgs.ToList());

PR Checklist

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• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
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• Breaking changes
  • None
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  • Documentation needed
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• User-facing changes
  • Not Applicable
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  • Documentation needed
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• Testing - New and feature
  • N/A or can only be tested interactively
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  • Make sure you've added a new test if existing tests do not effectively test the code changed
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[alerickson]

Closing this PR in favor of #728