[Performance improvements for find/install] Fix IEnumerable issues

[SydneyhSmith]

Summary of the new feature / enhancement

Misuse of IEnumerable is causing multiple server hits in 'Find'.

• 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());

Proposed technical implementation details (optional)

No response

[alerickson]

Resolved with PR #728