Refactor container image processing

The Image and Layer object structures have changed significantly:
- legacy parent_id and parent_digest attributes are removed from Image

- new attributes have been added to correctly track the tarball of an
  image or layer and its extracted location:
  - "extracted_location" is the absolute path where an image or layer is
    extracted
  - "archive_location" is the absolute path to an image or layer archive
  Therefore we have these attribute renames, additions and deletions:
    - Image.base_location -> Image.extracted_location
    - Image.archive_location: added
    - Layer.extracted_to_location -> Layer.extracted_location
    - Layer.layer_location -> Layer.archive_location
    - Layer.layer_sha256 -> Layer.sha256
  Also:
    - Layer.layer_id is now the sha256 of the Layer archive
    - Image.sha256, os_version, variant: added

- the layer_id is now based on the SHA256 of the layer tarball and not
  based on the UUID-like directory names that contain a "layer.tar" in
  Docker image.
- Image.config_digest is now prefixed with "sha256:"
- All mappings keys are now lowercased recursively, including for labels

- The way Image and Layers archives are extracted has changed
  significantly. Images are extracted as before keeping symlinks
  (which are essential to support certain Docker image layouts).
  In contrast, Layers are now exracted using extractcode and links are
  ignored.

- Layer/Image.to_dict() now accepts a new "exclude_fields" argument with
  a list of field names to optionally exclude.

- Add experimental support for container images using the OCI layout and
  add a new Image attribute "image_format" with the value "docker" or
  "oci"

- Add experimental support for Windows-based containers.

Signed-off-by: Philippe Ombredanne <pombredanne@nexb.com>