add `raco pkg catalog-archive`

The `catalog-archive` command not only copies a catalog, but also copies
all package sources referenced by the catalog to create a snapshot that
doesn't depend on any external pieces.

One reason you might want to do that is to download all packages on a
machine with general network access, and then building packages on a
(virtual) machine whose network access is limited.

This commit also adds support for relative package sources in the
catalog protocol, where a client converts a relative package source to
an absolute one.  Since the relative-to-absolute conversion happens on
clients, relative sources will be of limited use until v6.0 and v6.0.1
become deprecated.

A new `current-pkg-lookup-version` parameter corrects the misuse of
`current-pkg-scope-version` for package-catalog queries.