356561 Require that all children of a composite repository can be loaded

- Loading a composite repository now fails if any of its children
  cannot be loaded. So in case of transient network problems, a p2
  operation now fails fast, instead of generating obscure (and
  possibly severe, yet unnoticed) follow-up problems.
- p2 repository providers who rely on the old, lenient behaviour may
  re-enable it by adding the property 'p2.atomic.composite.loading'
  with value 'false' in their compositeContent.xml/
  compositeArtifact.xml.
- p2 clients who need to communicate with broken composite
  repositories which are out of maintenance can revert to the old
  behaviour by setting the system property
  eclipse.p2.atomic.composite.loading.default=false.

Bug: 356561 Composite Repositories have non-deterministic content by default
20 files changed