Move ASCII graph to --graph switch, and add a --tree alternative & fallback

[mosteo]

Currently we attempt to print the ASCII art box graph always at the end of alr with --solve. This has two problems: one, that when easy-graph isn't installed a warning is always emitted. Two, that when installed, that graph can quickly become too large and may difficult using the rest of the output of --solve (which may be the actual part one is interested in).

With this PR, this ASCII graph is removed from --solve and emitted stand-alone using the --graph switch, for when one is actually interested in that graph.

As a fallback for when easy-graph isn't available, a tree representation is provided:

xxx=0.0.0
├── hello=1.0.1 (^1)
│   └── libhello=1.0.1 (^1.0)
├── superhello=1.0.0 (*)
│   └── libhello=1.0.1 (~1.0)
├── unobtanium* (direct,missed) (*)
└── wip* (direct,linked,pin=/fake) (*)

This tree can also be obtained explicitly with with --tree. It shows in essence the same information than --solve, but because of the explicit branching, it may be more readable in that regard. --solve remains as the "text-only" more detailed output.