The primary purpose of this PR is to fix a failing (integration) test: CPAN_Download_Version_Succeeds. The test is failing because metacpan.org now returns Fastly JavaScript which causes CPANProjectManager.EnumerateVersionsAsync HTML-parsing to fail. This result of EnumerateVersionsAsync is used by PackageDownloader to select the largest version when the specified purl is missing a version.

This PR changes CPANProjectManager.EnumerateVersionsAsync to obtain the version list from fastapi.metacpan.org's JSON response (rather than parsing HTML.)

This PR also makes a small number of organizational changes:

• VS Solution folders for src and test with corresponding projects under each
• One small step towards idiomatic unit tests by introducing a new Shared.Lib.Tests unit testing project, where test organization matches the code under test.
• The new version metadata logic is in the new smaller, focused CPANMetadataClient class and adopts a few more modern conventions. The new code hopes for dependency injection one day, prefers CancellationToken over HttpClient.Timeout, and uses async over .Result.