This adds support for storing packages directly on Azure, with no truly
"local" (on-disk) repo used. The existing Azure PublishedStorage
implementation was refactored to move the shared code to a separate
context struct, which can then be re-used by the new PackagePool. In
addition, the files package's mockChecksumStorage was made public so
that it could be used in the Azure PackagePool tests as well.
Signed-off-by: Ryan Gonzalez <ryan.gonzalez@collabora.com>
