container: Add support for the dir transport

[lukewarmtemp]

This patch allows container image deployment stored in a local directory in addition to the supported remote OCI registry, OCI archive tarball, and local container storage. For example, the following command is now supported:

sudo ostree container image deploy --imgref ostree-unverified-image:dir:/var/home/core/fcos --sysroot /var/home/core/sysroot --stateroot fedora-coreos

The reason for making this change relates to the effort to create an FCOS qemu image from a container image input using OSBuild. OSBuild stores container image sources in a local directory in order to solve another bug and, thus, should not be changed. Therefore, to allow OSBuild to deploy from a container image, this patch is needed.

[cgwalters]

Minor style note; we tend to use "Linux kernel style" git commits here, which use a "topic
prefix". Also the subject should be imperative.

Something like:

container: Add support for the dir transport

Procedurally otherwise, I think this makes sense, with the caveat that

   dir:path
    An  existing local directory path storing the manifest, layer tarballs and signatures as individual files. This is a non-standardized format, primarily useful for debugging or non‐
  invasive container inspection.

• Please add some basic unit tests for parsing the transport
• It'd also be nice to add an integration test