Make pinning a stable feature

[florelis]

This removes the experimental feature gating the use of pinning to make it a stable feature. Credit to @ryfu-msft for this piece

It also adds a missing check for an edge case that the tests check. It wasn't being hit before because the pinning feature wasn't enabled for that test. Specifically, CompositeSource_NullAvailableVersion checks that we don't fail if the available version of a package is null; the pinning feature added checking for the pins after search, and when doing that it would access the available version without checking if it existed. This shouldn't have that much of an effect on users since we always keep populate the available version/source when we have an available package in the composite.

Microsoft Reviewers: Open in CodeFlow