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. A package is fetched from the archive with highest priority.

It also deprecates the concept of default archive in chisel.yaml.
However, the field can still be present and it will be parsed but
IGNORED.

DEPRECATED: "archives.<archive>.default" field in chisel.yaml.
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Co-authored-by: Alberto Carretero <[email protected]>