Add --dependencies and --offline options

[lydell]

This adds --dependencies and --offline options, just like elm-test-rs.

--dependencies oldest is useful for package development. This is implemented by reversing the list of available versions for a package.

I added --offline in the same PR, because elm-test-rs documents that --offline does not work with --dependencies and that made me think more about online and offline and what “newest” and “oldest” actually mean. To have --dependencies oldest and --dependencies newest work correctly and in non-surprising ways, I needed to rethink some caching and online/offline behaviors around dependency solving.

I also noted that the error message when a solve failed looked a bit out of context, and that we showed no progress output while solving online (slow). Before:

Because there is no version of elm/random in 1.0.0 <= v < 2.0.0 and elm-explorations/node-test-runner-example-package 1.0.0 depends on elm/random 1.0.0 <= v < 2.0.0, elm-explorations/node-test-runner-example-package 1.0.0 is forbidden.

After:

Solving dependencies
Failed to solve dependencies:
Because there is no version of elm/random in 1.0.0 <= v < 2.0.0 and elm-explorations/node-test-runner-example-package 1.0.0 depends on elm/random 1.0.0 <= v < 2.0.0, elm-explorations/node-test-runner-example-package 1.0.0 is forbidden.

[lydell]

It turned out that the OnlineAvailableVersionLister wasn’t doing anything online, and the OfflineAvaileableVersionLister was doing the same work, but wrongly only looked in one place (~/.elm/0.19.2/packages/, but not ~/.elm/pubgrub/). So I removed the “offline” one and renamed the “online” one.