This commit adds support for fetching packages from multiple archives.
It introduces a new field ``archives.<archive-name>.priority`` which
takes in a signed integer and specifies the priority of a certain
archive. This value is particularly useful when there are multiple
archives. A package is fetched from the archive with highest priority
regardless of the "default" archive, unless the slice definition file of
that package specifies a particular archive using the "archive" field.

Example chisel.yaml:

    format: v1
    archives:
	foo:
	    version: 22.04
	    components: [main, universe]
	    priority: 20
	    public-keys: [..]
	bar:
	    version: 22.04
	    components: [main]
	    default: true
	    priority: 10
	    public-keys: [..]

In the above example, if a package exists in both of the archives, the
package will be fetched from archive "foo" since it has higher priority.
Note that, archive "bar" is the default. However, if in a particular
package such as below, the "archive" field is used to specify a
particular archive, that particular archive will be fetched from.

    package: test-package
    archive: bar
    slices:
    	...

The above package will be fetched from archive "bar", regardless of
priority.

Reference: Specification RK018.

If an archive has a negative priority, ignore it while choosing the best
archive for a package. Unless, a package particularly asks for that
particular archive with negative priority. In that case, that archive is
chosen for the package.

This commit merges the changes of 'feat: create
manifest.wall'(canonical#142) into this
branch. We need the changes of this commit to include the updated
testing framework and changes.

Co-authored-by: Alberto Carretero <[email protected]>

This commit deprecates the concept of default archive in chisel.yaml.
In essence, it deprecates the "archives.<archive>.default" field from
the chisel.yaml configurations.

However, the field can still be present and it will be parsed but
IGNORED. (See setup_test.go)

DEPRECATED: "archives.<archive>.default" field in chisel.yaml.

There were some usage of the default field in archives left behind. This
commit cleans them up.