sui-move: [12/x][move-package/lock] Use DependencyGraph in download_dependency_repos

[amnn]

Another occasion where the previous implementation recursively traversed the transitive dependency graph -- the new approach discovers the graph and then iterates through its nodes to download them.

The second step is currently technically unnecessary, but becomes relevant in the presence of externally resolved dependencies where third-party resolvers can introduce sub-graphs into the DependencyGraph without having to explore them locally.

Test Plan

move/language/tools/move-package$ cargo nextest

Stack

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See also: #788 for main.