Signed-off-by: Dmitry Harkavyi <dmytro.harkavyi@sylabs.io>

Use github.com/CalebQ42/squashfs module to read the contents of
Singularity Image Format (SIF) container images without mounting them.

Expose Singularity Images via GetImageFromSource rather than adding a
SIF-specific GetSifImageFromSource by providing a GGCR v1.Image
implementation backed by a SIF. This avoids the requirement of separate
code paths within consumers of stereoscope such as syft.

Signed-off-by: Adam Hughes <stickmanica@gmail.com>

Switch to fork of github.com/CalebQ42/squashfs, which removes the
GPLv2-licensed indirect github.com/rasky/go-lzo dependency.

Signed-off-by: Adam Hughes <9903835+tri-adam@users.noreply.github.com>

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>

Modify the TestSimpleImage and TestImageSymlinks code to incorporate
testing of a singularity sif image source.

This requires signficant adaptations as singularity squashes
containers down to a single layer.

Singularity is expected to be available, and is is now installed in
the ci-bootstrap Makefile target (from a GitHub release).

Signed-off-by: David Trudgian <david.trudgian@sylabs.io>

Signed-off-by: Alex Goodman <alex.goodman@anchore.com>