It takes multiple sweeps.

Example:

• Install haskell-snippets.nvim and luasnip
• Delete them from rocks.toml
• Rocks sync

Expected behaviour:

• Both plugins, as well as the jsregexp dependency are pruned

Actual behaviour:

• Only haskell-snippets.nvim is pruned. luasnip can't be pruned yet, if haskell-snippets.nvim isn't pruned first, because it depends on it.
• It takes more than one call to Rocks sync to actually prune all dependencies.

Potential solutions:

• Analyze dependencies, then figure out the order in which to prune rocks.
• repeat <pruning logic> until <no more rocks that depend on installed rocks left in to_prune list>.