Support for compressed files was removed in tuf v0.10.x leaving behind
some vestiges like the test logic in test_repository_lib, which is
duplicated below and carries a redundant comment, and setting compression
on in generate_project_data.py

Signed-off-by: Joshua Lock <jlock@vmware.com>

tuf removed support for compressed metadata in v0.10.x, therefore it is
confusing to carry comments referring to compressed versions of metadata.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Switch to using the new abstract files and directories support in
securesystemslib by taking an object which implements
securesystemslib.storage.StorageBackendInterface in the Repository
constructor, passed in by tuf.repository_tool.create_new_repository() and
tuf.repository_tool.load_repository()

The Updater class in tuf.client.updater does not specify a storage backend
and instead allows the functions in securesystemslib to perform the
default action of instantiating a LocalFilesystemBackend, that is the
updater does not currently support abstract filesystem backends and always
defaults to using local storage.

Finally we drop support for tuf.settings.CONSISTENT_METHOD as it's not as
clear how different copying modes should work when the details of the
underlying storage are abstracted away.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Rather than check for the existence of metadata files before trying to
load them in _load_top_level_metadata, we should just try and load them.

This is more idiomatic Python through employing EAFP (Easier to Ask
Forgiveness than Permission) principles.

Signed-off-by: Joshua Lock <jlock@vmware.com>

The specification lists four fundamental roles: root, targets, snapshot
and timestamp. Loading a repository where those roles are not present
should not be supported, therefore convert debug messages on the absence
of metadata files for these fundamental roles into a RepositoryError
exception.

Signed-off-by: Joshua Lock <jlock@vmware.com>

Utilise the abstract files and directories support to enable generating
targets metadata for files which aren't necessarily locally accessible,
rather than requiring that metadata for non-local files be provided via
existing fileinfo structures.

Signed-off-by: Joshua Lock <jlock@vmware.com>

We need the recently released securesystemslib 0.15.0 or newer for
abstract storage support.

Signed-off-by: Joshua Lock <jlock@vmware.com>